242 



to 1000 times. If the string were placed in a perfect vacuum the 

 movement would be critically damped. Silver follows with 245 mm 

 deflection if the length be 4.85 cm. Copper requires a longer string 

 viz. of 5.5(3 cm and gives about the same deflection. Practically we 

 shall have to make our choice between aluminium, silver, or copper, 

 whenever we want a high sensitiveness for small potential differences 

 with a critically damped movement. From the formula we conclude 

 that with a given mateiial the thinner the string the higher will be 

 the sensibility for small potential differences. 



Finally we shall have to consider one other possibility for ren- 

 dering the voltsensibilily as high as possible. 



We take again the case of a perfectly relaxed string, clamped at 



the ends. If the weight P be uniformly distributed over the entire 



length /, we must use the formula (1). But if the string is loaded 



in the middle only with the weight l\ the deflection will be exactly 



twice as large : 



P I' 



h=~ . (17) 



EI 192 ^ ' 



If we put the string, of a length / in a stronger magnetic field 

 H' but of a very shoi-t length ). so as to make Hl^=H'X, and if 

 we suppose I to be very small as compai-ed wilh /, we shall come 

 very near the conditions represented by the last formula (17). Especially 

 if we use strings of not too small a diameter there will be scarcely 

 any difliculty of making the magnetic (ield 10— 20 times stronger 

 and the string 10 — 20 times longer than the field. In this case we 

 practically double the deflection, but at the same time the damping 

 will have become too great. The damping factor will have become 

 nearly 1.0 instead of 8/15. Hence the magnetic field must be made 

 1/2 times weaker. Finally the sensibility for small potential differences 

 will become only |/2 times greater. 



