In order to understand the working of influence machines, it is necessary to have a clear idea of what is meant by the word influence as used in an electrical sense. In the previous chapter we saw that a pith ball was attracted by an electrified body, and that when the ball touched that body it received a charge of electricity. We now have to learn that one body can receive a charge from another body without actual contact, by what is called “influence,” or electro-static induction. In Fig. 2 is seen a simple arrangement for showing this influence or induction. A is a glass ball, and BC a piece of metal, either solid or hollow, made somewhat in the shape of a sausage, and insulated by means of its glass support. Three pairs of pith balls are suspended from BC as shown. If A is electrified positively, and brought near BC, the pith balls at B and C repel one another, showing that the ends of BC are electrified. No repulsion takes place between the two pith balls at the middle, indicating that this part of BC is not electrified. If the charges at B and C are tested they are found to be of opposite kinds, that at B being negative, and that at C positive. Thus it appears that the positive charge on A has attracted a negative charge to B, and repelled a positive one to C. If A is taken away, the two opposite charges on BC unite and neutralise one another, and BC is left in its original uncharged condition, while A is found to have lost none of its own charge. If BC is made in two parts, and if these are separated while under the influence of A, the two charges cannot unite when A is removed, but remain each on its own half of BC. In this experiment A is said to have induced electrification on BC. Induction will take place across a considerable distance, and it is not stopped by the interposition of obstacles such as a sheet of glass. [Illustration: FIG. 2.--Diagram to illustrate Electro-static Induction.] We can now understand why an electrified body attracts an unelectrified body, as in our pith ball experiments. If we bring a positively charged glass rod near a pith ball, the latter becomes electrified by induction, the side nearer the rod receiving a negative, and the farther side a positive charge. One half of the ball is therefore attracted and the other half repelled, but as the attracted half is the nearer, the attraction is stronger than the repulsion, and so the ball moves towards the rod. [Illustration: FIG. 3.--The Electrophorus.]