Sir William Thomson's quadrant electrometer is the most perfect form of electrometer yet constructed, and the one usually employed in cable testing. It consists of a very thin flat aluminium needle spread out into two wings, and hung by a wire from an insulated stem inside a Leyden jar, which contains a cupful of strong sulphuric acid, the outer surface of which forms the inner coating of the Leyden jar. A wire stretched by a weight connects the aforesaid needle with this inner coating. A mirror, rigidly attached to this needle by a rod, serves to indicate the deflection of the needle by reflecting the image of a flame on to a scale. The needle hangs inside four quadrants, which are insulated by glass stems: each pair of opposite quadrants are in electrical connection. Above and below the quadrants two tubes, at the same potential as the needle, serve to screen it and the wires in connection with it from all induction except that produced by the four quadrants. Suppose the needle charged to a high negative potential (-), then if the quadrants are symmetrically placed, it will deflect neither to the right nor to the left, so long as the near quadrants are at the same potential. If one of these be positive relatively to the other, the end of the needle under them will be repelled from the negative quadrant to the positive one, and at the same time the other end of the needle will be repelled from in the opposite direction. This motion will be indicated by the motion of the spot of light reflected by the mirror, and the number of divisions which the spot of light traverses on the scale measures in an arbitrary unit the difference of potential between the + and - quadrants.
The reflecting electrometer being a very delicate instrument, requires careful handling, and should only be used by a practised electrician. Its use would therefore be restricted to important stations, and special tests of a delicate nature.
Thomson's Reflecting Galvanometer.—A galvanometer is an instrument intended to detect the presence of a current and measure its magnitude.
The most sensitive galvanometer as yet constructed is the reflecting galvanometer of Sir William Thomson, a diagram of which is shown at [Fig. 84].
A small piece of magnetised steel watch spring, 3/8ths of an inch long, is fastened with shellac on the back of a little round concave mirror, and of about the size of a fourpenny piece. This is suspended by a piece of unspun silk thread in the centre of a coil of many hundred turns of fine copper wire insulated with silk, and well protected between the turns with varnish. The two ends of the coils are soldered to terminal screws a, b, so that any conducting wire can be joined up to it as required. The little mirror hangs in the middle of its coil, with the magnet lying horizontally. By means of a lamp L placed behind the screen, the light of which passes through a slit M, and is thrown on the face of the mirror, a spot of light is reflected on the scale N.
When a current passes through the coil, the little magnet is deflected, and since the magnet is attached to the mirror, which is very light, both are deflected as forming one body, and the spot of light moves accordingly along the scale N.
A powerful steel magnet S is placed above the coil, and can be moved up or down, whereby the directive force of the earth may be increased or weakened. This magnet S is used to steady the spot of light, which otherwise would shake about, and there would be no certainty about the measurement. A second magnet T is placed perpendicular to the magnetic meridian, to adjust the zero of the instrument, i.e., to bring back the spot of light to a fiducial mark at the centre of the scale when no current is passing.
This instrument should only be used at important stations, and when special tests of a delicate nature are required to be applied.
Astatic Galvanometer.—An astatic galvanometer is that in connection with which an astatic needle is employed, by the use of which the sensitiveness of a galvanometer is greatly increased.