( 253 ) 



S = 0.239 X - gnimmes (40) 



o 



We see from these formulae that the tension is inversely pi-oport- 

 onal to the sensitiveness. 



For a sensitiveness c = 1 the tension would be 239 milligranimes. 

 Assuming with Threlfall ^) that a thin quartz thread has a tensile 

 strength of 100 kilogrammes per m]\P section, wdien using a string 

 of 2.39 fi^ section or 1.75 ft diameter, the sensitiveness of the galvano- 

 meter may be diminished to c = \, i.e. to 1 mm. deflection for 

 1 micrampere without the thread breaking. The strongest tension we 

 applied with string n". 14 corresponds to a sensitiveness of 1 mm. 

 deflection for 3 X 'iO-' Amp., hence c =. 3.3, while the diameter of 

 the string amounts to 1.7 ft. 



From these data it appears that the maximum tejision used by us 

 is still 3 times smaller than the tensile strength of the string. We 

 remark here that this limit has only been calculated for the quartz, 

 while the silver which might also contribute something to the strength 

 has been left out of account. 



String n°. 10, Avhen uncoated, has a diameter of 2.4 /i. From this 

 w^e calculate that it will bear such a tension that the sensitiveness 

 of the galvanometer is reduced to a minimum of 6',„i„ = 0.529. The 

 maximum of the practically applicable sensitiveness is c'„„x = 10\ 



The ratio ^-^ =: 1.89 X i^^' indicates the possible variation in scnsi- 



tivéness, which may undoubtedly be called enormous. 



The value found for Cmax gives rise to some remarks about the 

 corresponding tension >S'inin. According to formula (40) we should 

 find for c = 10^ a tension of 2.39 X ^0"*^ grammes, an absurd value, 

 since the weight of string n". 10, here used as an example, amounts 

 to 6.85X10"*' grammes, i.e. nearly three times more, and since of 

 course the tension in a vertically stretched string cannot possibly 

 be less than its weight. But this absurd result is easily explained by 

 remembering that formula (40) only holds if the ({uartz thread is 

 strongly stretched and so behaves as a string, w-hich was premised 

 in the calculation of the tension. 



From the results obtained we must conclude that with feeble 

 tension the quartz thread no longer moves like a sti'ing. There are 

 sufficient data to prove that the motion of the quartz thread does 

 not even completely agree with the vibration of a string when the 



1) Philosoph. Magaz. Vol. 30 (.j), p. 99. 1890. 



