=236. The Toepler-Holtz Induction Machine.=--This is a type of induction or influence machine that is often used for producing a continuous supply of electricity as in the operation of "X" ray machines, in lecture demonstrations, etc. This machine (Fig. 214) consists of two discs: one fixed, the other mounted so as to revolve. Upon the back of the fixed plate are two sectors of tinfoil which become charged oppositely. Upon the revolving plate are six metallic discs. These discs act like the discs of the electrophorus. They become charged by induction from the charges upon the sectors fastened to the fixed plate. The brushes held by a rod touch the discs at just the right time to take off the repelled charge. The charges induced upon the discs are taken off by two metal combs whose points are held close to the revolving disc. The Leyden jars assist in accumulating a good strong charge before a spark passes between the terminal knobs. Some machines are built up of several pairs of plates and give correspondingly large amounts of electricity. [Illustration: FIG. 214.--The Toepler-Holtz induction machine.] Important Topics _Static Electric Generators._--(a) plate friction machine, (b) electrophorus, (c) induction or influence machine. Exercises 1. Potential is similar to what other terms that we have studied? 2. What three electrical phenomena are better understood from a study of the lines of force? 3. How many charges may be produced by an electrophorus before the plate needs to be electrified again? Explain. 4. The static induction machine is often called a "continuous electrophorous." Why? 5. The Leyden jars used with the induction machine cause much brighter sparks to be produced than without them. Explain. 6. With the Leyden jars removed, would the frequency with which the sparks pass between the knobs be increased or decreased? Explain. 7. Mention three likenesses and three differences between magnetism and static electricity. 8. Will you receive a greater shock by touching a knob of a charged Leyden jar when it is held in the hand or when it is standing on a sheet of glass? Explain. 9. In what way may an electric charge be divided into three equal parts? Review Outline: Magnetism and Static Electricity Comparison between Magnetism and Static Electricity. Substances are: { magnetic, { conductors, { non-magnetic. { insulators. Produced by: induction. friction, or induction. Theory: molecular. electron. (fluid) { attraction, { attraction, Fields of Force { repulsion, { repulsion, Explain: { induction, { induction. { action of compass.