Bench Degree·ELECTROMAGNETISMchapter

Appendix A: The Bench

The whole volume’s kit in one place. Every experiment in this book is on this list, so you should never have to stop mid-chapter and wait two days for a delivery. Each chapter names what it needs when it needs it and points back here; if you would rather order once and be done, order from this list.

Chapter 1 costs about fifteen dollars and contains the entire argument of the book. A coil of wire, a magnet and an LED, and the LED lights with nothing plugged into anything.

The starter set that carries you through Chapters 1 to 8, which is the whole physics of the subject and both of its motors, is about $100, and nearer $65 if you already own a multimeter and a kitchen scale. Everything after that is optional in the strict sense: the second half of the book can be read with no further purchase, and the reason to spend the money is that Chapters 10 to 13 are much better felt than read.

Nothing here is specified by brand or by part number. Every item is described by function, range or gauge, so the list stays orderable from anyone, anywhere, for as long as the book exists. Suppliers change and a printed link dies permanently. A specification does not.

One purchase deserves naming twice, because it is the safety architecture of this whole volume. A doorbell or thermostat transformer, about $14, with a 16 V AC output. Nine experiments in this book run on it. Its iron core isolates everything you touch from the mains supply, which is the only reason a beginner can work with alternating current on a table at all.

Section 1: The starter set, for Chapters 1 to 8

Starter set total: about $100, or nearer $65 if you own a meter, a scale and a junk drawer.

Items you will want and probably already have: a cordless drill (Chapter 7), a phone (Chapters 13 and 15), a metal biscuit tin and kitchen foil (Chapter 5), an FM radio (Chapter 5), an extension cord and an electric kettle or heater (Chapter 2), and a wine glass (Chapter 14).

Section 2: Added for Chapters 9 to 17

Second set total: about $260, or about $190 without the coil build in Chapter 10 and the resonant pair in Chapter 12.

Everything, both sections, comes to about $360.

Section 4: The one expensive thing, and whether you need it

A way to see a waveform. Chapter 2 is about the shape of alternating current, and a shape is difficult to argue about in prose.

You have three routes and they cost $2, $25 and $50.

A computer sound card and free oscilloscope software, plus two 10:1 dividers built from 100 kilohm and 10 kilohm resistors, about $2. This is genuinely enough for Chapter 2. It shows you a sine wave, a rectified sine, and the difference between them, which is the point of the chapter. Its ceiling is about 20 kHz and a few volts, so it stops being useful the moment you reach resonance.

A DSO138 kit scope, about $25. Single channel, 200 kHz, and you solder it together yourself. It covers Chapter 10’s resonance work and Chapter 12’s 100 kHz coils, which is where the sound card gives up.

A combined USB scope and signal generator, about $50. Two channels and a sweepable source in one box. This is the one to buy if you intend to do Chapters 10 to 13, because those chapters are about finding a resonance, and finding a resonance means driving a circuit and watching it answer at the same time.

The second expensive item is a variable frequency drive, $60 to $90 for a half-horsepower unit, which makes three-phase from a single-phase outlet. Chapter 8’s last experiment is the only thing in this volume that needs it, and it is also the only experiment in this volume where getting it wrong is genuinely dangerous. If you have never worked inside a motor terminal box, do that one beside somebody who has, or borrow ten minutes on a machine at a workshop and watch the two wires swap. The lesson survives being watched.

Section 5: What you already own more of than you think

Every motor in your house is Chapter 7 and Chapter 8. The refrigerator compressor, the furnace blower, the pool pump, the bench grinder, the extractor fan and the washing machine are single-phase induction motors, and the nameplate on each one tells you its pole count if you know how to read it. Chapter 7 teaches that in fifteen minutes and it costs nothing.

A cordless drill is Chapter 6, a brushed DC motor with a commutator, and the same drill is also Chapter 7’s rotating field generator when you put a magnet in the chuck.

Your phone is two instruments in this book. It has a three-axis magnetometer, so Chapter 15 measures the earth’s field and your fridge magnet in the same units Tesla is named after. It also has a wireless charging coil, which is Chapter 12’s entire subject, and a camera with a long-exposure mode, which is how Chapter 13 recreates the most famous photograph in electrical engineering.

Every phone charger and laptop brick is Chapter 3, a transformer and a rectifier in a plastic case, and a dead one is a free source of ferrite cores.

A microwave oven contains the highest voltage in your house, roughly 2,000 V from its own transformer, and a capacitor that has killed people who assumed the plug being out was enough. Chapter 3 reads its nameplate. Nothing in this book asks you to open one.

An extension cord is a resistor, and a 30 m (100 ft) run of 16 AWG with a kettle on the end will show you several volts of loss, which is the whole argument of Chapter 9 happening in your hallway.

A biscuit tin is a Faraday cage, an AM radio is a receiver, and a barbecue lighter is a spark-gap transmitter. Chapters 5 and 11 use all three, and the total cost is three dollars.

Section 6: The experiments, and where they are

Forty-two, and the whole list fits on the kit above.

The five in bold are the ones to do if you do only five. The first delivers the whole thesis for fifteen dollars. The second shows you that a transformer’s ratio is a count of turns and nothing more. The third makes a falling magnet behave like a falling feather, for ten dollars, and it is the best ten dollars in this book. The fourth builds a field that rotates with nothing moving, which is the idea the twentieth century ran on. And the fifth teaches you to read a photograph, which is a laboratory skill and costs nothing to acquire.

Seventeen of the forty-two cost nothing at all beyond a meter and what is already in the house, and four of them need only a calculator, a notebook or a patent. This is a cheap subject to learn honestly. The expensive half of it, which is the utility grid and the industrial motor, is deliberately left to be read and measured rather than built.

Section 7: Kits

A single collection covering the starter set and the Chapter 10 coil, ordered once, is the obvious convenience and it is being worked on. When it exists it will be at benchdegree.com/kit/electromagnetism, and that address is deliberately the only place it will be published: suppliers and product listings change, and a link printed in a book cannot be corrected. The specifications above can be taken to any supplier and will still make sense in twenty years.

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