Volta's voltmeter

Voltage is measured in volts. 220 volts is quite a strong and dangerous voltage. It is not surprising that in the US and Japan the voltage used is half that value, that is 110 volts. As a result, however, much thicker cables have to be used - for the same electric power we need twice as strong electric current. That is why there are so many cables on each post in Japan.

The unit of voltage comes from the name of Aleksander Volta. Voltage is measured with a voltmeter. But how did Volta measure the voltage if there had been no voltmeters before his times? As you can see in this photo, he used a common glass bottle which was square in section to make taking of the readings easier.

You can easily construct such a voltmeter. You will need a glass, a piece of aluminium foil and a nail. A school ruler can be the source of voltage if you rub it against a woollen sleeve. Technically, such a device would be called an electroscopes