shaped metal pieces (Fig. 3), which are supported on insulating pillars. In many of the older types of electrometers these pillars are of glass coated with shellac, but this insulation is very imperfect, and in the more modern instruments is replaced by supports of ebonite or amber. The needle is suspended by a quartz fibre, or a bifilar silk suspension, and is adjusted so as to be symmetrically placed with respect to the quadrants when they are metallically

connected together and FIG. 4.-Quadrant electro

therefore at the same

meter needle.

potential. The position of the needle is read by observing the position of a spot of light reflected by a small mirror attached to the needle upon a scale placed at a suitable distance from it. From the needle a platinum wire with a light piece of platinum. foil attached to it dips into a glass vessel containing sulphuric acid, which is thus in connection with the needle and increases the

electric capacity of the system. At the same time the platinum foil dipping into the acid serves to damp the motion of the needle when set in motion, and prevents it from swinging backwards and forwards before coming to rest. If the needle is sufficiently light, however, the sulphuric acid may be dispensed with.

To use the instrument the needle is charged to a high potential, and the opposite quadrants are connected by wires. When these two opposite pairs of quadrants are brought to different potentials a couple is exerted on the needle, which tends to draw it into one or other pair of quadrants. This couple is opposed by the twist on the fibre suspending the needle, which eventually takes up a new position of equilibrium depending on the difference of potential between the quadrants. If the instrument has been carefully adjusted the deflection is proportional to the difference of potential between the quadrants, provided that the needle is displaced only through a small angle.

There are other methods of using the instrument, but the one just given is usually