A First Course in Physics · Ch. XV, §§ 394–403 · Chapter 15: Effects of Electrical Currents
45. Coils and electro-magnets: the bell and the telegraph
Goal: Your student can explain why a current loop acts like a magnet, find the N pole of a coil with the right hand, build an electro-magnet, and explain how an electric bell and a telegraph work.
You'll need
- The book, open to these pages
- A large iron nail or bolt, about 2 m of thin insulated wire (enameled or plastic-coated), an AA battery and tape
- Paper clips or staples
- A compass
- A notebook and pencil
- A calculator (optional, for the problems)
What's in the book
A loop carrying a current behaves like a flat magnet; how it turns in a magnetic field; a helix (coil) acts like a bar magnet; the right-hand rule for coils; the electro-magnet and its strength in ampere-turns; the electric bell; Morse's telegraph, relays and sounders. Nine problems follow.
The lesson, step by step
Hook question
SayJunkyard cranes lift whole cars with a magnet, then drop them on command. How can a magnet be switched off?
Read together
Read §§ 394–403. The rule for a coil:
If the coil is grasped in the right hand in such a way that the fingers point in the direction in which the current is flowing in the wires, the thumb will point in the direction of the north pole of the helix
Try it: build an electro-magnet
Wind the wire tightly around the nail 30–50 times, leaving long ends, and strip or sand the ends bare. Touch the ends to the battery: the nail picks up paper clips. Let go: they drop (§ 398). Count how many clips you pick up with 20 turns and with 50 turns: more ampere-turns, stronger magnet (§ 399). Use the compass to find which end is N, and check it with the right-hand rule.
Connect only for a few seconds at a time; the wire and battery get warm. Safety: use only 1.5-volt AA or AAA batteries for every circuit in this course. Never connect anything to a wall outlet, and don't hold a bare wire straight across a battery for more than a few seconds (it gets hot and drains the battery).
Explain the bell
Trace Fig. 312: the current magnetizes the core, which pulls the armature and hammer; that breaks the contact, the magnet lets go, the spring pulls back, the contact closes again, and so on, many times a second (§ 400).
The telegraph
Read § 401 and § 403. Tap out a message in Morse code with a flashlight or by knocking. The first line, from Baltimore to Washington, opened on May 24, 1844, with "What hath God wrought?" (§ 401). The relay (§ 402) lets a weak current from far away switch on a strong local one: an idea still used in every computer.
Say it back
Your student explains why a relay needs many turns of fine wire (problem 8).
Problems
Work problems 1–9 on book page 305.
Afterwards
Find electro-magnets at home: a doorbell, the latch of a washing machine, the speaker in a phone or radio.
Answers to the book's problems
1. Its north face is toward the south, so it will turn half way around (through 180°) until that face points north. 2. It turns until its plane is at right angles to the lines of force between the magnet poles. 3. In opposite directions; counterclockwise around the N pole, as you look down at it. 6. About 0.037 ampere, assuming one 150-ohm relay at each end (100 volts ÷ (2000 + 300 + 400) ohms). 7. About 370 ampere-turns. 8. About 0.042 ampere × 500 turns, only about 21 ampere-turns; on a long, high-resistance line the current is tiny whatever the magnet, so a relay must have very many turns (hence high resistance) to get enough ampere-turns. 9. From east to west.
Worked out for this site from the scan. If a number in your copy differs, trust the book.