( 262 ) 



Calling c, the sensitiveness for a potential difference, expressed in 

 millimetres deflection per microvolt, we have 



C =: Cs w 



from which follows, together with formula (49) that 



cs = — X10'.tX^>T, (50) 



We further derive from formulae (46) and (48) that 



niAi' 8 X 10' 



^■=p^x~^ <") 



These last two formulae (50) and (51) furnish us with all the 

 data for easily examining the influence of various changes in the 

 galvanometer on its sensitiveness for thermo-currents. 



In the first place we point out that making a thread thinner or 

 thicker has no influence on the sensitiveness Cs, if only the pi-oduct 

 m-^iv in formula (51) remains unaltered. 



Using a metal wire, the value of n\iv remains naturally the same 

 however the thickness of the ^vire may ^ ary, if always wires of the 

 same metal and of the same length are used. It may be advantageous 

 to use a heavy, thick wii-e, since then the air-damping may be 

 neglected, without the wire requiring to be placed in a vacuum. 

 Also the practical difficulties of applying a very feeble tension may 

 perhaps in this Ciise be more easily solved by means of an elastic 

 stretching arrangement than when a thin wire is used. 



In the second place we point out that by formula (50) the sensi- 

 tiveness Cs is inversely proportional to the intensity of the field and 

 to the length of the wire. 



We first give our attention to the intensity of the field and 

 imagine a thread of constant length /= 12.7 cm. The question how 

 far the intensity of the field may under these conditions be dimin- 

 ished, can be answered by means of formula (51). 



In order to raise the sensitiveness to a maximum, the field strength 

 must be reduced to a minimum. If Tj and / are constant, then 

 according to (51) m^iv must be made a mijiimum. Using a thread 

 of homogeneous material, m^tv is only determined by the nature of 

 the material, so that the question about the minimum of 7/ is reduced 

 to the question for what material m^v) is a minimum. As far as I 

 can judge this is the case for aluminium, which has for /= 12.7 cm. 

 a value of 7??i?ü,i/ = 1.394 X 10 ~'. 



Assuming for T^ the value 2.5 seconds, the deflection has been 

 nearly completed after 10 seconds. A distance of 1.85 °/„ of the total 

 deflection remains to be travelled through then. After 12.5 sec. this 



