on Galvanometers. 89 



EXPLANATION OF TABLE. 



r= resistance in ohms. 



M = millimetres deflexion per micro-ampere when the period is 

 10 seconds, and the scale is placed as in the actual use of 

 the instrument. 



D = deflexion in scale-divisions per micro-ampere when the period 

 is 10 seconds, and the scale-distance is equal to 2000 scale- 

 divisions. 



S = swing produced per micro-coulomb under the same conditions as 

 in last. 



V= volume occupied by the convolutions of wire, in cubic centi- 

 metres approximately. 



I = moment of inertia of the suspended system, in C.Gr.S. units 

 approximately. 



DI = deflexion in scale-divisions (scale-distance equal to 2000 scale 

 divisions) per micro-ampere for constant controlling mo- 

 ments, and for a periodic time equal to 10 VI seconds. 



Sl2 = swing per micro-coulomb under the same conditions as in last. 



Columns 9, 10, and 11 give the deflexion per micro-ampere, and 

 the swing per micro-coulomb, when the period is 10 seconds 

 and the resistance of each instrument one ohm. 



Columns 12 and 13 give the deflexion per micro-ampere and the 

 swing per micro-coulomb, for the same controlling moment, 

 and for the resistance of each instrument equal to one ohm. 



Columns 14 and 15 give the deflexion per micro-ampere and the 

 swing per micro-coulomb per cubic centimetre of coil, when 

 the period is 10 seconds and the resistance of each galvano- 

 meter one ohm. 



Columns 16 and 17 give the deflexion per micro -ampere and the 

 swing per micro-coulomb per cubic centimetre of coil, for 

 the same controlling moment, and the resistance of each 

 galvanometer equal to one ohm. Periodic time of any 

 instrument is 10 Vl seconds. 



C.I., Central Institution. 



F.T.C., Finsbury Technical College. 



f The constants of these instruments have been kindly furnished 

 by the makers. 



X These instruments were tested after being in use for some time 

 and without their needles being remagnetized. 



* These numbers are calculated from those given in column 3. 



of maximum sensibility when used for these experiments on sulphur. 

 Granting this, and looking at the observations given on page 472 of the 

 Phil. Mag., vol. xxviii., from which the constant of the galvanometer was 

 determined, we find that there was considerable vagueness of the zero, 

 amounting in some cases to more than 4 per cent, of the double deflexion, 

 as well as a gradual change in the position of the zero from 105 to 78. 

 If, then, the control were diminished to about ^ of the value it then had, 

 and which would be necessary to give the galvanometer the sensibility 

 claimed for it, the vagueness of the zero would probably become too 

 serious for the instrument to be used for any but the very roughest 

 measurements. 



