This was another remarkable discovery, and Faraday proceeded to put it into another different shape. He wound a coil of wire round a bar of soft iron, and connected the ends with a galvanometer. Nothing happened so long as the bar of iron was not magnetised. But on suddenly placing the bar of soft iron between the two poles of a magnet, the bar became magnetised, and the galvanometer gave a momentary start. He removed the magnet, and the galvanometer gave a start in the opposite direction. Faraday expressed this by saying that a momentary current is induced in a coil of wire by starting or stopping a current in a neighbouring coil, or by creating or destroying a magnet near the coil. Such induced currents are also called “induction currents.” They are familiar to many through the “induction coils” which are used to give electric shocks. In these induction coils there is a cylinder of iron surrounded by a coil of copper wire, and another coil of very fine wire round that again. A current is started and stopped in rapid succession in the first coil, and each starting and stopping induces a momentary current in the outer coil. These momentary currents are made to travel through the person who holds the handles. It is they that produce the curious pricking sensation. [Illustration: Fig. 67.--Induced Currents.] Instead of sending a current through one coil inside another, we may take a coil through which a current is already passing, and put it inside another coil connected with a galvanometer. Each time the current-bearing coil is put in or taken out, the needle of the galvanometer moves, showing that a momentary current is passing through it. When we consider that great cities are now lighted by means of these induced electric currents, it will be understood what a vast importance Faraday’s discovery bore within itself. When we pass the electric light works, a “central station” as it is called, we hear the throbbing of the engines which turn the machines designed to produce the electric current. These machines are called “dynamos.” A dynamo consists of two things. One of them is a great magnet which produces a powerful magnetic field. The other is a set of coils of wire which, driven by a steam engine or gas-engine, move through the magnetic field. [Illustration: Fig. 68.--A Simple Dynamo.] Each coil, as it moves in and out of the magnetic field, becomes the bearer of a momentary induction current, for to move a coil through a magnetic field produces the same effect as to make or unmake an electromagnet near the coil, or to start or stop a current in a neighbouring coil.