on Galvanometers. 11 



careful consideration must be given to the suspension-wires 

 which lead the current into and out of the moving coil, since 

 being usually made of German silver they may easily become 

 slightly heated by the relatively large current employed with 

 a very low resistance-galvanometer, and in consequence 

 their elasticity temporarily diminished. This w T ould of course 

 have the effect of making the instrument more sensitive 

 for large currents than for small; but as the sensibility 

 for large currents would, until a limit was reached, increase 

 with the time the current was kept flowing through the gal- 

 vanometer, such an instrument could not be used for accurate 

 measurement. One method of overcoming this difficulty is to 

 make the suspension of very thin phosphor-bronze strip, such 

 as is employed in the construction of delicate " Ayrton and 

 Perry magnifying-springs." For a thin strip has, for its 

 cross-section, a very small torsional rigidity and a very large 

 radiating surface, and therefore is the very thing to employ 

 when we want a conductor with small torsional rigidity, and 

 which will be very slightly raised in temperature by the 

 passage of a current through it. 



D'Arsonval galvanometers, when used as voltmeters, are 

 subject to the error that affects all electromagnetic voltmeters 

 arising from a variation of the resistance of the coil due 

 to a variation in the temperature of the room ; if, however, 

 both the coil and the suspension-wires are made of platinum- 

 silver, a very curious compensating effect is brought about, 

 since the percentage increase of resistance with temperature 

 of platinum-silver is almost exactly equal to the percentage 

 diminution in its torsional rigidity. Hence when a rise of 

 temperature diminishes the current flowing through the 

 instrument for a given P.D. maintained at its terminals and so 

 diminishes the deflecting couple, a proportional diminution in 

 the torsional rigidity of the suspending wires, and therefore 

 in the controlling couple, is produced at the same time. It is 

 thus possible to construct an electromagnetic voltmeter of one 

 metal only, which has no temperature error. 



VII. Sensibilities of Different Types of Galvanometers. 



Apart from differences in detail (as, for example, in the size, 

 shape, or number of the coils, &c), galvanometers may be 

 divided into four distinct classes, viz.: — 



1. Galvanometers in which the moving system consists of one 



or more magnetic needles turning about an axis at right 

 angles to the magnetic axes of the needles. 



2. Galvanometers in which the needles have a motion of 



