( 252 ) 



1 If the magnetic field is noii-lioinogeiieoiis, there may arise 

 vortex curreiils in the hiyer of silver during a deflection of the 

 (juartz thread. 



2. If the siring is para- or diamagnetic, it may by its motion 

 induce currents in the iron of the pole shoes. 



3. Also a non-magnetic string will induce a movement of electricit}' 

 in the pole shoes when it is passed by a current and mo\"es. 



But all three causes are so small 'that they may probably be 

 neglected compared with the air-damping. 



8. The fciision of the quart: thread. 



S. 



In order to calcidate the tension of the quartz thread under various 

 circumstances, we begin with assuming a special case, namely that 

 the tliread is strongly stretched and is placed over its entire length 

 in a homogeneous magnetic field. A constant current passed through 

 the galvanometer causes a' permanent deflection of the thread which 

 assumes the shape of a catenary. 



Calling ?/i the deflection of the middle of the thread and i^ the 

 pondeiomotive force experienced by the tiiread, the tension is: 



S='^ (36) 



Here S and /\ are expressed in dynes, while the deflection k^ and 

 the length / are given in centimetres. 



KONV 



^ = ^n (37) 



'i 



ill which (\, denotes the sensitiveness of the galvanometer, as was 



already menlioned with f(:)rmula (29), expressed in centimetres 



deflection of the middle of the thread per dyne. 



From formulae (36) and (37) follows that 



•S=^ (38j 



And from (31) and (38) we derive that the tension is 



S = J^ (39, 



Substituting again for II, I and b their values, viz. H = 17600, 

 /=:12.7 and ^ = 660, we find .5'= 234 X -7 tb'»es, which result, 

 calculated in grammes, gives for the value of the tension 



