Fig. 5.
The first electric machine consisted of a ball of sulphur fastened to a spindle which could be turned by a crank. By holding the hands or a pad of silk upon the revolving ball, electricity was produced.
Fig. 6.
Fig. 7.
9. The Cylinder Electric Machine consists, as shown in Fig. 6, of a glass cylinder so mounted that it can be turned by a crank. Friction is produced by a pad of leather C, which presses against the cylinder as it turns. Electric sparks can be taken from the large "conductors" which are insulated from the earth. The opposite electricities unite with sparks across D and E. If use is to be made of the electricity, either the rubber or the prime conductor must be connected with the ground. In the former case positive electricity is obtained; in the latter, negative.
10. The Plate Electrical Machine. Fig. 7 also shows an old form of machine. Such machines are made of circular plates of glass or ebonite, two rubbing pads being usually employed, one on each side of the plate. One operator is seen on an insulated stool (Fig. 7), the electricity passing through him before entering the earth by way of the body of the man at the right.
