A S the writer had occasion to maintain potentials from about 5,000 volts on, which were kept up by a Toepler-Holtz machine driven by an electric motor, more constant than was possible with an ordinary point-overflow to earth, he designed the following simple arrangement, which answered the purpose very well. A vertical projection of the regulating device is shown in the figure. A light disk A of polished metal is suspended by means of Fig. 1.-B an insulating rod from a suitable support so that

more »

... it can rotate with the least possible friction about a horizontal axis B. Opposite A is mounted another disk C, through small holes in which project a great number of sharp needles fastened into a third disk D. Extending downward from A is a, rod carrying at its lower end a light metallic plate, which serves at the same time to damp the motion of A and to make electrical connection between it and some slightly