Idiostatic and Heterostatic Instruments

14. In the gold-leaf electroscope, the only electrification in the field is the electrification to be tested. In the Quadrant Electrometer the needle is kept always charged.

Instruments in which the only electrification is that which we wish to test, are called Idiostatic. Those in which there is electrification independent of that to be tested are called Heterostatic. In an idiostatic instru- ment, like the gold-leaf electroscope, the indications are the same, whether the potential to be tested is positive or negative, and the amount of the indi- cation is, when very small, nearly as the square of the difference of potential.

In a heterostatic instrument, like the quadrant electrometer, the indication is to the one side or to the other, as the potential is positive or negative, and the amount of the indication is proportional to the difference of potentials, and not to the square of that difference.

Hence an instrument on the heterostatic principle, not only indicates of itself whether the potential is positive or neg- ative, but when the potential is very small its motion for a small variation of potential is as great as when the potential is large, whereas in the gold-leaf electroscope a very small electrification does not cause the gold leaves to separate sensibly.

In Thomson’s Quadrant Electrometer there is a contrivance by which the potential of the needle is adjusted to a constant value, and there are other organs for special purposes, which are not represented in the figure which is a mere diagram of the most essential parts of the instrument.

Insulators.

15.] In electrical experiments it is often necessary to support an electrified body in such a way that the electricity may not escape. For this purpose, nothing is better than to set it on a stand supported by a glass rod, provided the surface of the glass is quite dry. But, except in the very driest weather, the surface of the glass has always a little moisture condensed on it. For this reason electrical apparatus is often placed before a fire, before it is to be used, so that the moisture of the air may not condense on the warmed surface of the glass. But if the glass is made too warm, it loses its insulating power and becomes a good conductor.

Hence it is best to adopt a method by which the surface of the glass may be kept dry without heating it.

The insulating stand in the figure consists of a glass vessel C, with a boss rising up in the middle to which is cemented the glass pillar a a. To the upper part of this pillar is cemented the neck of the bell glass B, which is thus supported so that its rim is within the vessel C, but does not touch it. The pillar a carries the stand A on which the body to be insulated is placed.

In the vessel C is placed some strong sulphuric acid c, which fills a wide shallow moat round the boss in the middle. The air within the bell glass B, in contact with the pillar a, is thus dried, and before any damp air can enter this part of the instrument, it must pass down between C and B and glide over the surface of the sulphuric acid, so that it is thoroughly dried before it reaches the glass pillar. Such an insulating stand is very valuable when delicate ex- periments have to be performed. For rougher purposes insulating stands may be made with pillars of glass varnished with shellac or of sealing-wax or ebonite.

Fig. 3.

16.] For carrying about an electrified conductor, it is very convenient to fasten it to the end of an ebonite rod. Ebonite, however, is very easily elec- trified. The slightest touch with the hand, or friction of any kind, is sufficient to render its surface so electrical, that no conclusion can be drawn as to the electrification of the conductor at the end of the rod. The rod therefore must never be touched but must be carried by means of a handle of metal, or of wood covered with tinfoil, and before making any experiment the whole surface of the ebonite must be freed from electrification by passing it rapidly through a flame.

The sockets by which the conductors are fastened to the ebonite rods, should not project outwards from the conductors, for the electricity not only accumulates on the projecting parts, but creeps over the surface of the ebonite, and remains there when the electricity of the conductor is discharged. The sockets should therefore be entirely within the outer surface of the conductors as in Fig. 4.

It is convenient to have a brass ball (Fig. 4), one inch in diameter, a cylin- drical metal vessel (Fig. 5) about three inches in diameter and five or six inches deep, a pair of disks of tin plate (Figs. 6, 7), two inches in diame- ter, and a thin wire about a foot long (Fig. 8) to make connection between electrified bodies. These should all be mounted on ebonite rods (penholders), one eighth of an inch in diameter, with handles of metal or of wood covered with tinfoil.