Wl I. I.I AM SIEMENS, /-.A'..V. 37 



tin conductor is immersed, in connection with the other pole 

 of the battery, including the galvanometer coil into the circuit. 

 We then get a deflection, and from this deflection we calculate 

 the resistance by the simple formula of the resistance being 

 equal to the sine of the angle, divided by the sine of the angle 

 of deflection, which would be produced by an unit of electro- 

 motive force, or by one element upon the same needle, multiplied 



by the number of elements employed, or R = n^ %-. The 



Sin p 



deflection which is produced by an element upon the needle has 

 to be determined frequently in testing by this method, and is 

 called the constant of the instrument. 



This method is not so simple as the Wheatstone method, where 

 we compare simply one resistance with another ; it has, however, 

 the advantage of greater sensitiveness, being much improved in 

 this respect by the application of Professor Thomson's reflecting 

 galvanometer, which with its mirror attached to an extremely 

 delicate needle, reflects a ray of light upon a large scale, and 

 enables us to read with clearness very small angles. For very 

 small angles, as you are aware, the sine may be taken for the 

 angle itself, and therefore, in measuring with this instrument, 

 instead of taking the sines of the angle, we take the simple 

 deflection, and thus save calculation. 



DIFFEKENTIAL METHOD. "Within the last twelvemonth, we 

 have devised and introduced into practice at the Charlton works, 

 another method of measuring resistances, in which neither artificial 

 resistances nor a sine galvanometer are used, and which recom- 

 mends itself by its extreme facility of operation. The instrument 

 employed consists of a very delicate galvanometer, and of a coil 

 put separately upon a screw carriage, which latter acts upon the 

 same needle under the influence of a constant current. The 

 resistance to be measured is put into the galvanometer circuit with 

 a battery of fifty cells, to produce the current. We connect a 

 second battery to the balancing coil, which is mounted on a slide, 

 that admits of being moved to and fro. It is evident that if I 

 know the effect that would be produced by one cell on the move- 

 able coil, and of a hundred cells on the stationary or galvanometer 

 coil in balancing each other upon the instrument itself, I shall be 

 able to judge the effect also of a larger resistance in the circuit of 



