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.

The electroscope measures the level of body "electrisation". The most frequently used is so-called "needle electroscope". When the collector at the top (and the indicator inside) get charged, the movable needle is repelled from the fixed one. The bigger charge is - the bigger deviation of the needle. Such a simple electroscope you can construct on your own, using a glass, a toothpick and a piece of aluminium foil.

The simplest electroscope (photo on the right).

To measure the "electricity", or more exactly the electrical charge, simply rub a plastic stick and than touch the upper part of the aluminium strips. When they get charged, the two strips repel each other, like two Christmas balls attract each other when connected to a piezoelectric lighter. The two strips accumulate the same sign charge, while the balls opposite charges.

To de-charge the electroscope, simply touch it. Your body has much bigger electrical capacity, so when the same charge is shared between you and the electroscope, the electrical potential is smaller. This is the point: Volta's electroscope measures what we call today "voltage".

Another Volta's invention was an electrophore, a "toy", using the electrostatic induction, apparently creating the electric charge out of nothing. Its construction is simple but the operating procedure consists of several steps: touching the plastic plate, grounding the upper surface, moving away, in a repeated cycle.