---
title: "Current Intensity and the Ammeter"
book: "Primary & Middle School Physics"
subject: physics
language: en
chapter: 55
exercises: 12
source: https://one-course.com/books/physics/1/en/chapter/55-current-intensity-and-the-ammeter
---

# Chapter 55 — Current Intensity and the Ammeter

“Strong current, weak current” — we have talked this way for years, dosing brightness and grip by feel. Feel retires today. The current’s strength gets a name, a [unit](https://one-course.com/books/physics/1/en/chapter/37-measurement-in-science-units-and-instruments#def-g6-measurement-in-science-measuring) and an instrument; and the first thing the instrument proves is a law you have long suspected but never once measured.

## 55.1 Intensity and its unit

**Definition 55.1 (Intensity).**

The *intensity* $I$ of an [electric current](https://one-course.com/books/physics/1/en/chapter/47-circuits-loops-series-parallel#def-g7-circuits-series-parallel-current) [measures](https://one-course.com/books/physics/1/en/chapter/37-measurement-in-science-units-and-instruments#def-g6-measurement-in-science-measuring) how strong the march is: how much electric charge parades past a point of the circuit each second — the current’s flow rate, as a river’s is water-per-second. Its [unit](https://one-course.com/books/physics/1/en/chapter/37-measurement-in-science-units-and-instruments#def-g6-measurement-in-science-measuring) is the *ampere* ($\mathrm{A}$), honoring a great pioneer of electricity’s laws; everyday circuits often speak in *milliamperes*: $1\,\mathrm{mA} = 0.001\,\mathrm{A}$, a thousandth.

**Example 55.2 (Amperes to know by sight).**

A little indicator lamp sips about $0.02\,\mathrm{A}$ — $20\,\mathrm{mA}$; a flashlight [bulb](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-bulb), around $0.3\,\mathrm{A}$; the great lamp of a car’s headlight, $5\,\mathrm{A}$; a kettle at full roar, $10\,\mathrm{A}$; a starting car engine gulps $100\,\mathrm{A}$ from its [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery); a lightning stroke, for its millisecond, rams tens of thousands of [amperes](#def-g8-intensity-ammeter-intensity) down its channel. And the danger scale is sobering: currents of a few hundredths of an [ampere](#def-g8-intensity-ammeter-intensity) across a human body can already be fatal — respect begins well below one [ampere](#def-g8-intensity-ammeter-intensity).

**Remark 55.3 (The person and the unit).**

Written out, the [unit](https://one-course.com/books/physics/1/en/chapter/37-measurement-in-science-units-and-instruments#def-g6-measurement-in-science-measuring) is lowercase — “a current of two [amperes](#def-g8-intensity-ammeter-intensity)” — while the capitalized word names the person, as with every [unit](https://one-course.com/books/physics/1/en/chapter/37-measurement-in-science-units-and-instruments#def-g6-measurement-in-science-measuring) honoring a scientist. The symbol $\mathrm{A}$ stays capital. You will meet this convention again shortly, twice.

## 55.2 The ammeter

**Definition 55.4 (Ammeter).**

An *ammeter* [measures](https://one-course.com/books/physics/1/en/chapter/37-measurement-in-science-units-and-instruments#def-g6-measurement-in-science-measuring) [intensity](#def-g8-intensity-ammeter-intensity). It counts the march that passes *through itself* — so it must be inserted *[in series](https://one-course.com/books/physics/1/en/chapter/23-series-circuits#def-g4-series-circuits-series)*, into the very loop it [measures](https://one-course.com/books/physics/1/en/chapter/37-measurement-in-science-units-and-instruments#def-g6-measurement-in-science-measuring), like a tollbooth on the road. Built to disturb the march as little as possible, it opposes the current almost not at all — which is also why an ammeter misconnected straight across a [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) is a [short circuit](https://one-course.com/books/physics/1/en/chapter/48-short-circuits-and-electrical-safety#def-g7-short-circuits-safety-device) with a dial: the classic way to break one.

**Method 55.5 (Measuring an intensity).**

1. switch the circuit off; *break* the loop where the current is to be counted;
2. insert the [ammeter](#def-g8-intensity-ammeter-ammeter) into the gap: current in by its $\mathrm{A}$ [terminal](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) , out by its *common* [terminal](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) , so the conventional direction enters the meter’s front door;
3. on a multi-range meter, start on the *largest* range, then step down until the reading sits comfortably on the dial — protecting the meter from surprises;
4. close the [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch) , read — and write the [unit](https://one-course.com/books/physics/1/en/chapter/37-measurement-in-science-units-and-instruments#def-g6-measurement-in-science-measuring) .

Backward connection on a needle meter slams the needle against its stop; digital meters merely show a minus sign, forgiving students daily.

![The ammeter takes its place in the loop: a tollbooth in series, counting the march that lights the lamp.](https://one-course.com/images/onecourse/chapters/physics-1/g8-intensity-ammeter/fig-78f1f0ddd2fa.svg)

*The [ammeter](#def-g8-intensity-ammeter-ammeter) takes its place in the loop: a tollbooth [in series](https://one-course.com/books/physics/1/en/chapter/23-series-circuits#def-g4-series-circuits-series), counting the march that [lights](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) the lamp.*

![The workbench reality: one instrument, two probes, and the circuit’s current becomes a number on the display.](https://one-course.com/images/onecourse/chapters/physics-1/g8-intensity-ammeter/img-2e97af0cb47a.jpg)

*The workbench reality: one instrument, two probes, and the circuit’s current becomes a number on the display.*

## 55.3 The first measured law

**Proposition 55.6 (One loop, one intensity).**

In a [series circuit](https://one-course.com/books/physics/1/en/chapter/23-series-circuits#def-g4-series-circuits-series), the [intensity](#def-g8-intensity-ammeter-intensity) is the same at every point of the loop. Move the [ammeter](#def-g8-intensity-ammeter-ammeter) anywhere — before the lamp, after it, beside the [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery): the reading does not change. What five years of equal-brightness [bulbs](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-bulb) suggested, the meter now states in numbers: nothing is consumed en route; one road, one march, one $I$.

![Three tollbooths on one loop: before, between and after the lamps, every ammeter reports the same 0.30\, A.](https://one-course.com/images/onecourse/chapters/physics-1/g8-intensity-ammeter/fig-439940ae50b0.svg)

*Three tollbooths on one loop: before, between and after the lamps, every [ammeter](#def-g8-intensity-ammeter-ammeter) reports the same $0.30\,\mathrm{A}$.*

**Example 55.7 (Reading the law’s fine print).**

The law says *along one loop*, not *whatever you do to the loop*. Add a second lamp [in series](https://one-course.com/books/physics/1/en/chapter/23-series-circuits#def-g4-series-circuits-series) and the march weakens everywhere at once — perhaps $0.30\,\mathrm{A}$ drops to $0.17\,\mathrm{A}$ — but it weakens *equally* everywhere: the three tollbooths agree on the new number too. One loop, one [intensity](#def-g8-intensity-ammeter-intensity) — whatever that [intensity](#def-g8-intensity-ammeter-intensity) currently is.

**Example 55.8 (A glance at the junction).**

Set tollbooths on a parallel pair instead: main road $0.50\,\mathrm{A}$; [branch](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) one $0.30\,\mathrm{A}$; [branch](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) two $0.20\,\mathrm{A}$. The suggestive sum is no accident — the marches into a [junction](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) and out of it must balance, since charge is neither created nor destroyed at a fork in the road. The full law, with its partner for voltage, holds court in the chapter after next; your meters have already met it.

**Remark 55.9 (The multimeter).**

The workshop’s yellow brick — the *multimeter* — folds the [ammeter](#def-g8-intensity-ammeter-ammeter) and several other instruments into one box with a selector dial. Dial on $\mathrm{A}$ or $\mathrm{mA}$, leads in the marked sockets, and it is your [ammeter](#def-g8-intensity-ammeter-ammeter), series rules and all. Two chapters of this book live inside it already; a third moves in next chapter — with, beware, *opposite* wiring rules.

## 55.4 Exercises

**Exercise 55.1 ★.**

What does [intensity](#def-g8-intensity-ammeter-intensity) [measure](https://one-course.com/books/physics/1/en/chapter/37-measurement-in-science-units-and-instruments#def-g6-measurement-in-science-measuring), in river-language? Give its [unit](https://one-course.com/books/physics/1/en/chapter/37-measurement-in-science-units-and-instruments#def-g6-measurement-in-science-measuring) and symbol, and convert $250\,\mathrm{mA}$ to [amperes](#def-g8-intensity-ammeter-intensity).

**Solution of Exercise 55.1.**

The current’s flow rate — charge passing per second, as a river counts water per second. [Unit](https://one-course.com/books/physics/1/en/chapter/37-measurement-in-science-units-and-instruments#def-g6-measurement-in-science-measuring): the [ampere](#def-g8-intensity-ammeter-intensity), $\mathrm{A}$; $250\,\mathrm{mA} = 0.25\,\mathrm{A}$.

**Exercise 55.2 ★.**

Match the [intensity](#def-g8-intensity-ammeter-intensity) to its scene: $0.02\,\mathrm{A}$; $0.3\,\mathrm{A}$; $10\,\mathrm{A}$; $100\,\mathrm{A}$ — kettle, indicator lamp, starting engine, flashlight [bulb](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-bulb).

**Solution of Exercise 55.2.**

$0.02\,\mathrm{A}$: indicator lamp. $0.3\,\mathrm{A}$: flashlight [bulb](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-bulb). $10\,\mathrm{A}$: kettle. $100\,\mathrm{A}$: starting engine.

**Exercise 55.3 ★.**

Why must an [ammeter](#def-g8-intensity-ammeter-ammeter) be inserted [in series](https://one-course.com/books/physics/1/en/chapter/23-series-circuits#def-g4-series-circuits-series)? What is it built to oppose almost not at all, and what accident does that make possible?

**Solution of Exercise 55.3.**

It counts only the march passing through itself, so it must stand in the road — [in series](https://one-course.com/books/physics/1/en/chapter/23-series-circuits#def-g4-series-circuits-series). It opposes the current almost not at all; wired straight across a [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) it therefore becomes a [short circuit](https://one-course.com/books/physics/1/en/chapter/48-short-circuits-and-electrical-safety#def-g7-short-circuits-safety-device) with a dial.

**Exercise 55.4 ★.**

List the four steps of a proper [intensity](#def-g8-intensity-ammeter-intensity) measurement. Why start on the largest range?

**Solution of Exercise 55.4.**

[Switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch) off and break the loop; insert the meter with the current entering by the $\mathrm{A}$ [terminal](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) and leaving by common; start on the largest range and step down; switch on, read, write the [unit](https://one-course.com/books/physics/1/en/chapter/37-measurement-in-science-units-and-instruments#def-g6-measurement-in-science-measuring). The largest range first, so an unexpectedly strong current meets the sturdiest setting instead of wrecking a delicate one.

**Exercise 55.5 ★.**

State the one-loop-one-intensity law. Which old equal-brightness observation did it turn into numbers?

**Solution of Exercise 55.5.**

In a [series circuit](https://one-course.com/books/physics/1/en/chapter/23-series-circuits#def-g4-series-circuits-series) the [intensity](#def-g8-intensity-ammeter-intensity) is the same at every point of the loop. It turned into numbers the old observation that two identical series [bulbs](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-bulb) glow exactly equally, wherever they sit.

**Exercise 55.6 ★.**

An [ammeter](#def-g8-intensity-ammeter-ammeter) before a series [motor](https://one-course.com/books/physics/1/en/chapter/41-electric-circuits-and-safety#ex-g6-circuits-and-safety-motor) reads $0.45\,\mathrm{A}$. What will it read after the [motor](https://one-course.com/books/physics/1/en/chapter/41-electric-circuits-and-safety#ex-g6-circuits-and-safety-motor)? And beside the [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery)?

**Solution of Exercise 55.6.**

$0.45\,\mathrm{A}$, and $0.45\,\mathrm{A}$: one loop, one [intensity](#def-g8-intensity-ammeter-intensity).

**Exercise 55.7 ★.**

A second lamp joins a series loop and the [ammeter](#def-g8-intensity-ammeter-ammeter)’s $0.30\,\mathrm{A}$ becomes $0.17\,\mathrm{A}$. Has the law failed? Explain the fine print.

**Solution of Exercise 55.7.**

No. The law promises one value *around the loop at a given moment*, not the same value after the loop is rebuilt. The second lamp weakened the march — equally everywhere: all tollbooths now agree on $0.17\,\mathrm{A}$.

**Exercise 55.8 ★.**

A digital [ammeter](#def-g8-intensity-ammeter-ammeter) reads $-0.25$ $\mathrm{A}$. What has the student done, and how grave is it?

**Solution of Exercise 55.8.**

Wired the meter backward — current entering by the common [terminal](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery). On a digital meter, harmless: the minus sign is the whole penalty (a needle meter would have slammed its stop).

**Exercise 55.9 ★★.**

Main road $0.60\,\mathrm{A}$; [branch](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) one $0.35\,\mathrm{A}$. What does [branch](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) two carry, and by what reasoning? What does the second [junction](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel)’s tollbooth read?

**Solution of Exercise 55.9.**

$0.60 - 0.35 = 0.25\,\mathrm{A}$ — the [junction](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) must balance: what flows in flows out. The second [junction](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel)’s tollbooth reads the reunited $0.60\,\mathrm{A}$.

**Exercise 55.10 ★★.**

A student wires the [ammeter](#def-g8-intensity-ammeter-ammeter) *[in parallel](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel)* with the lamp “to compare them”. Predict the two consequences — for the lamp, and for the meter — and name the villain the student has accidentally built.

**Solution of Exercise 55.10.**

[In parallel](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel), the near-effortless meter is a bypass: the lamp, deserted, dims to nothing, while the meter carries a rushing current limited by almost nothing — a [short circuit](https://one-course.com/books/physics/1/en/chapter/48-short-circuits-and-electrical-safety#def-g7-short-circuits-safety-device) through the instrument, blowing its internal [fuse](https://one-course.com/books/physics/1/en/chapter/48-short-circuits-and-electrical-safety#def-g7-short-circuits-safety-fuse) if fortune smiles. The student built the villain of the safety chapter.

**Exercise 55.11 ★★.**

Two identical [bulbs](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-bulb) [in parallel](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) each carry $0.30\,\mathrm{A}$. The [battery’s](https://one-course.com/books/physics/1/en/chapter/48-short-circuits-and-electrical-safety#def-g7-short-circuits-safety-battery) main road carries how much? A third identical [bulb](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-bulb) joins [in parallel](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel): predict the main road’s new burden, and name what drains faster.

**Solution of Exercise 55.11.**

The main road carries the sum: $0.60\,\mathrm{A}$. A third identical [branch](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) brings it to $0.90\,\mathrm{A}$ — and the [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery), serving three full marches, drains fastest of all.

**Exercise 55.12 ★★★.**

Design the full measurement plan for a [two-branch](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) [parallel circuit](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) (lamp and [motor](https://one-course.com/books/physics/1/en/chapter/41-electric-circuits-and-safety#ex-g6-circuits-and-safety-motor)): how many [ammeter](#def-g8-intensity-ammeter-ammeter) positions are needed to know *every* current in the circuit; the minimum number of measurements that suffices (one may be deduced — by which balance?); and the order of operations that never leaves a meter wired dangerously.

**Solution of Exercise 55.12.**

Currents to know: the main road’s $I$, and each [branch](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel)’s ($I_1$, $I_2$) — three in all (the loop segments beyond the [junctions](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) repeat these). Two measurements suffice: [measure](https://one-course.com/books/physics/1/en/chapter/37-measurement-in-science-units-and-instruments#def-g6-measurement-in-science-measuring) any two, and the [junction](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) balance $I = I_1 + I_2$ hands over the third. Safe order: circuit off; insert the meter into the chosen road; largest range; on; read; off before moving the meter — and the meter never, at any step, bridges two points of different roads.

## 55.5 Problem: The Metrology Bench

**Problem 55.1.**

Weekend problem — qualifying day at the electronics workshop; four stations, one ammeter each; the apprentice’s license

To earn workshop privileges, each apprentice tours four measurement stations. Bring the chapter.

**Part I — Station one: the simple loop.** [Battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery), [switch](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#ex-g3-first-electric-circuit-switch), one lamp.

1. Describe, step by step, inserting the [ammeter](#def-g8-intensity-ammeter-ammeter) to [measure](https://one-course.com/books/physics/1/en/chapter/37-measurement-in-science-units-and-instruments#def-g6-measurement-in-science-measuring) the lamp’s current.
2. The reading: $280\,\mathrm{mA}$ . Express it in [amperes](#def-g8-intensity-ammeter-intensity) .
3. The examiner moves your meter to the [battery’s](https://one-course.com/books/physics/1/en/chapter/48-short-circuits-and-electrical-safety#def-g7-short-circuits-safety-battery) other side. Predict the reading, citing the law.
4. “And if I had two [ammeters](#def-g8-intensity-ammeter-ammeter) in this loop at once?” Where could they sit, and what would they show?

**Part II — Station two: series pair.** Two different lamps, A brighter than B, [in series](https://one-course.com/books/physics/1/en/chapter/23-series-circuits#def-g4-series-circuits-series).

5. An apprentice predicts: “Brighter A carries more current than dim B.” Test the prediction with the law — and reconcile the equal currents with the unequal brightness (what must differ between the lamps?).
6. The meter reads $150\,\mathrm{mA}$ between A and B. Write down every other current in the loop.
7. Lamp B is [short-circuited](https://one-course.com/books/physics/1/en/chapter/48-short-circuits-and-electrical-safety#def-g7-short-circuits-safety-device) by a clip lead. Predict the new reading and both lamps’ behavior.
8. The examiner asks for one sentence on why the workshop forbids clip leads across the *[battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery)* while permitting the demonstration across B.

**Part III — Station three: the [junction](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel).** A lamp [branch](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) and a [motor](https://one-course.com/books/physics/1/en/chapter/41-electric-circuits-and-safety#ex-g6-circuits-and-safety-motor) [branch](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) [in parallel](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel).

9. Main road: $0.52\,\mathrm{A}$ ; lamp [branch](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) : $0.30\,\mathrm{A}$ . The [motor](https://one-course.com/books/physics/1/en/chapter/41-electric-circuits-and-safety#ex-g6-circuits-and-safety-motor) [branch](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) ’s current, by balance?
10. The [motor](https://one-course.com/books/physics/1/en/chapter/41-electric-circuits-and-safety#ex-g6-circuits-and-safety-motor) jams (still connected, not turning) and its [branch](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) climbs to $0.40\,\mathrm{A}$ . Assuming the lamp [branch](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) keeps its $0.30\,\mathrm{A}$ , what does the main road carry now — and which workshop guard is listening?
11. The lamp is unscrewed. Predict all three tollbooth readings.
12. Why does the [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) drain fastest with both [branches](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) healthy — where is generosity paid?

**Part IV — The license.**

13. Write the apprentice’s pledge: three lines — where an [ammeter](#def-g8-intensity-ammeter-ammeter) belongs and never belongs; what one loop guarantees; what a [junction](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) balances.

**Solution of Problem 55.1.**

**1.** Off; break the loop between lamp and [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery); insert the [ammeter](#def-g8-intensity-ammeter-ammeter), current in by $\mathrm{A}$, out by common; largest range, step down; on; read; write the [unit](https://one-course.com/books/physics/1/en/chapter/37-measurement-in-science-units-and-instruments#def-g6-measurement-in-science-measuring). **2.** $0.28\,\mathrm{A}$. **3.** $280\,\mathrm{mA}$ again — one loop, one [intensity](#def-g8-intensity-ammeter-intensity): the tollbooth counts the same march on either side. **4.** Anywhere on the loop — both [in series](https://one-course.com/books/physics/1/en/chapter/23-series-circuits#def-g4-series-circuits-series), any positions: both show $280\,\mathrm{mA}$. **5.** The law vetoes the prediction: series lamps carry the identical current. The difference lies in the lamps themselves — unequal builds turn the same march into unequal [light](https://one-course.com/books/physics/1/en/chapter/3-light-and-shadows#def-g1-light-and-shadows-source) (the next chapters name what differs). **6.** Every point of the loop: $150\,\mathrm{mA}$ — before A, after B, beside the [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery). **7.** B dies (deserted by the bypass) and the loop, having lost one opposer, carries a stronger march: the meter climbs — and lamp A brightens accordingly. **8.** “Across B the bypass still leaves lamp A guarding the loop; across the [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery) it would leave nothing at all — and an unguarded march is the fire chapter, not a demonstration.” **9.** $0.52 - 0.30 = 0.22\,\mathrm{A}$. **10.** $0.40 + 0.30 = 0.70\,\mathrm{A}$ — and the [branch](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) [fuse](https://one-course.com/books/physics/1/en/chapter/48-short-circuits-and-electrical-safety#def-g7-short-circuits-safety-fuse) (or breaker) guarding the [motor](https://one-course.com/books/physics/1/en/chapter/41-electric-circuits-and-safety#ex-g6-circuits-and-safety-motor)’s line is listening closely: jammed [motors](https://one-course.com/books/physics/1/en/chapter/41-electric-circuits-and-safety#ex-g6-circuits-and-safety-motor) are how guards earn their keep. **11.** Lamp [branch](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel): $0\,\mathrm{A}$; [motor](https://one-course.com/books/physics/1/en/chapter/41-electric-circuits-and-safety#ex-g6-circuits-and-safety-motor) [branch](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel): its own $0.40\,\mathrm{A}$ unchanged (parallel independence); main road: $0.40\,\mathrm{A}$. **12.** Generosity is paid at the [battery](https://one-course.com/books/physics/1/en/chapter/16-a-first-electric-circuit#def-g3-first-electric-circuit-battery): two healthy [branches](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) drink two full marches, and the summed main-road current empties the store fastest. **13.** For example: “An [ammeter](#def-g8-intensity-ammeter-ammeter) stands in the road, never across it. One loop guarantees one [intensity](#def-g8-intensity-ammeter-intensity) — everywhere, always. And a [junction](https://one-course.com/books/physics/1/en/chapter/30-series-and-parallel-circuits#def-g5-series-parallel-circuits-parallel) balances its books: marches in equal marches out, to the milliampere.”