( 260 ) 



the string galvaiiometer come out least. Besides the difïïcnlty of the 

 air-damping one meets here that of tlie electromagnetic damping, 

 Avhich soon becomes very considerable. 



With unchanged strength of the field the electromagnetic damping 

 is inversely proportional to the ohmic resistance of the circuit. With 

 thread n" 10 the air-damping and electromagnetic damping are as 

 1 : 0.6, the ohmic resistance in the closed circuit being 10.000 ohms. 

 When measuring an insulation resistance the electromagnetic damping 

 vanishes and the thread will want about 15 seconds for a deflection, 

 when the sensitiveness is regulated at 1 mm for 10~^^ amp. When 

 measuring a thermo-current, for which the external resistance in the 

 circuit may be neglected and only the galvano metric resistance of 

 10.000 ohms has to be reckoned, for the same sensitiveness the 

 duration of a deflection will l)e 1.6 times greater i.e. 24 seconds. 

 Putting the condition tiiat the duration of a deflectioji shall not 

 exceed 15 seconds, one has to be contented with a 1.6 times less 

 sensitiveness and obtains 1 mm deflection for 1.6 X 10 -^^ amp. or for 

 1,6X10-" volt. 



Since a deflection of 0.1 mm can still be observed in practice, the 

 now existing galvanometer will be able to show a P. D. of 1.6 X 10 -^ 

 volt when thermo-currents are measured. 



The application of a vacuum would only little increase the sen- 

 sitiveness for small potential differences, and would not reduce 

 the minimum to more than 0.6 x 10-^ volt. Also using a quartz 

 thread with smaller resistance will only cause little change in 

 this sensitiveness. If the ohmic resistance becomes n times less the 



smallest observable difference of potential ^vill become 



(1 + -) 



XlO-8 volt. 



But there are two means for increasing the sensitiveness for a 

 potential difference, which must be mentioned. They consist in making 

 the strength of the field smaller and in shortening the quartz thread. 



We suppose the string to be placed in a vacuum so that the 

 dami)iug of its motion is only caused by electromagnetic influences. 

 It is further assumed that tlie deflections are aperiodic and so slow 

 that the iiifluence of the mass of the string on its velocity of motion 

 may be neglected. If under these conditions the strength of the field 

 is reduced (L times, and at the same time the tension of the string 

 (f times, for an equal duration of a deflection tlie sensitiveness will 

 be (I times increased. 



But it is easy to show that a useful diminution of the intensity 

 of the field cannot be dri\'en very far. For it must be remembered 



