. The practical telephone handbook and guide to the telephonic exchange . A mirror galvanometermust be kept very steady,as the slightest jar will setthe mirror quivering, sothat readings cannot betaken. For this reason,the instrument is usually placed on a heavy slab ofFig. 427.—Simple Form of r J . Kelvin Galvanometer * stone, which rests on india-rubber blocks set on a structure built into a solid wall. The advantages of the Kelvin instrument over the ordinaryform of galvanometer are that the turns of wire can be broughtvery near to the needles, and so act powerfully on the astaticsystem, an
. The practical telephone handbook and guide to the telephonic exchange . A mirror galvanometermust be kept very steady,as the slightest jar will setthe mirror quivering, sothat readings cannot betaken. For this reason,the instrument is usually placed on a heavy slab ofFig. 427.—Simple Form of r J . Kelvin Galvanometer * stone, which rests on india-rubber blocks set on a structure built into a solid wall. The advantages of the Kelvin instrument over the ordinaryform of galvanometer are that the turns of wire can be broughtvery near to the needles, and so act powerfully on the astaticsystem, and a shorter length of wire is required for the samenumber of turns. The beam of light also forms a very longand weightless index, which moves twice as far as an actualfixed index of the same length, by reason of the law of reflection. &Arsonval Galvanometer.—This is a reflecting instrumentwhich to a large extent has displaced the Kelvin galvano-meter for many purposes. It is very convenient owing toits dead-beat action (which means that the deflection is made. ELECTRICAL MEASUREMENTS 453 direct to the proper point on the scale without unnecessaryvibration about that point) and its freedom from the effectsof external vibration and magnetic influences. It is not,however, so sensitive, and has not the all-round usefulnessof the Kelvin instrument. Fig. 428 shows its the poles of a strong compound horseshoe magnetis fixed a soft-iron cylinder, which serves to direct the lines offorce, and in the space between this and the poles of themagnet a rectangular coil of wire, with mirror attached, issuspended by a very thin silver wire at top and bottom, thes e
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Keywords: ., bookcentury1900, bookdecade1900, booksubjecttelephone, bookyear19
