bench · at the benchproves 1.1, target 2

Two nails in a glass of water

We think water conducts. Yet at 9 V, two nails in tap water pass only a few milliamps. A pinch of salt, and the current climbs five to twenty-five times. The water has not changed; what it carries has. In a wire, the carriers are electrons; in water, ions, of both signs, moving both ways. The bubbles will tell which way.

multimeter in mA

You need

From the drawer: the multimeter, a 9 V battery and its snap connector, two plain steel nails, 6 to 8 cm long, some cardboard, a 250 mL glass, salt. Nothing to buy.
No stainless, no galvanized: stainless does not dissolve, the chloride oxidizes in its place, with chromium thrown in; zinc dissolves before iron and hides the rust. Galvanized if you must.

Do

BeforeThe multimeter in series, red lead in the mA jack, 200 mA range; the fuse on that jack, its rating printed beside it, blows past that. At 100 mA the battery feels nothing; shorted, it delivers amps and is burning hot within a minute. As long as the iron of the plus nail is dissolving, no chlorine; if a swimming-pool smell rises: air the room, don’t lean over it, don’t leave it running for hours. The hydrogen at the other nail is a trace. Hands out of the water while it is connected; 9 V does nothing, but the habit starts here. The water, with its iron and maybe a hint of bleach: down the drain.

A piece of cardboard caps the glass, pierced with two holes 2.5 cm apart; the nails go through, 3 cm into the tap water. Battery plus to nail A; nail B to the red lead; COM to minus. At ten seconds: 1 to 5 mA in soft water, 5 to 15 in city water, up to 25 in hard well water. Switch to the 20 mA range for the hundredths, to 2 mA only if the reading is under 2.00. Write down I1.
Back to 200 mA. A pinch of salt, what two fingers hold, a tenth of a gram; stir with a wooden stick until dissolved, read: 20 to 80 mA, five to twenty-five times I1, more if the water is soft. Write down I2. Fine bubbles at nail B.
Two more pinches, stir, read: 60 to 180 mA, about three times I2. Write down I3. Near 180 mA, pull the nails up a centimetre for the fuse’s sake; the battery would deliver amps.
Watch for a minute. Nail B, at minus, foams with hydrogen; nail A, at plus, almost nothing, but the water around it turns yellow, then green, and a cloud of rust sinks. Swap the wires on the nails: bubbles and rust change nails, the reading hardly moves.

What proves the target

The photo: one nail foaming, the water yellowing under the other, the multimeter on I3, the sheet with the three values. The sentence you write, with your own numbers: "Water alone passes 5 mA, one pinch 40 mA, three pinches 110 mA. What conducts is not the water, it is what you dissolve in it: ions."

Pure water has almost no carriers, a ten-millionth of a mole of ions of each sign per litre. Tap water has a few, calcium, magnesium, bicarbonates, chlorides: that is I1. Salt splits into Na⁺ and Cl⁻, two carriers of opposite sign: the positives head for the minus nail, the negatives for the plus nail, and both count in the same direction. Conductivity rises with concentration; that is I2 over I1, then I3 over I2. The glass sees only 8 V; the reactions at the nails take the rest.

The electrons do not cross the water. They arrive by the wire at the minus nail and stop there; the water takes them, 2 H₂O + 2 e⁻ → H₂ + 2 OH⁻: the foam. At the plus nail there is a shortage; the iron gives them up, Fe → Fe²⁺ + 2 e⁻, more readily than water or chloride would: little gas, and iron that stains the glass. Conventional current enters the water at plus and leaves at minus; at each nail, a reaction joins the two kinds of carrier. This is an electrolytic cell; the battery in the freezer is the same one, run backwards.

What it serves

1.1 Charge and its movement

Target 2: Name the charge carriers by medium: electrons in a metal, ions in an electrolyte (the cell, the pack, salt water), electrons and holes in a semiconductor, ions and electrons in an arc or a plasma, and say which way each one goes.

What next?

1.1 Charge and its motion: the table of carriers, the cell.
The battery in the freezer: the same cell, delivering, its ions slowed by the cold.
6.2 The pack: lithium ions between two electrodes, the same trick.

object

Two nails in a glass of water

You need

Do

What next?

The EGO pack opened

Electricity before Franklin

The Leyden jar

Franklin’s fluid

Galvani’s frog

The pencil lead and the lighter

The battery in the freezer

Why does a contact get hot?

Why does a battery weaken in the cold?

1.1 Charge and its movement