Recording Thermometers for Clinical Work. 541 



This point has been emphasized by both Guye and Schuster, 

 and comes out clearly in Heavi side's investigation. 



The advantage which can be gained by making n large 

 and m small is not very great, because when 7i = m = l, 



c /C = (R-R / )/2(G + R / X 



or the sensitiveness is still half as great as in the limiting 

 case. 



For practical purposes it is more important to observe that 

 if n is small or m large, the sensitiveness may be indefinitely 

 reduced. This arrangement should always be avoided if 

 possible, as shown by the following example. 



Maxwell's Rule. 



3. Maxwell has given a rule which is very often quoted 

 and applied (' Electricity and Magnetism,' vol. i. section 

 348) . " Of the two resistances, that of the battery and that 

 of the galvanometer, connect the greater resistance so as 

 to join the two greatest to the two least of the four other 

 resistances." 



This rule is seldom applicable to accurate resistance 

 measurements, and when applied in platinum thermometry 

 has often led to disastrous results. A simple numerical 

 example will make this clearer. 



Suppose that it is required to measure the resistance of a 

 platinum thermometer, R = 10 ohms, with a post-office box 

 having ratio arms 10 and 1000 ohms. Suppose that the 

 galvanometer has a resistance Gr=10 ohms, and that the 

 battery is a 2 volt cell of negligible resistance. According 

 to Maxwell's rule we should connect the galvanometer so as 

 to join the two resistances of 1000 ohms to the two resistances 

 of 10 ohms, in which case we should have n = l, and ??i = 100 

 in fig. 1. Suppose that the balance is slightly disturbed by 

 a change dR=R — R'. Substituting in equation (1) we 

 obtain : 



rase (A), Maxwell's Rule, 



n=l, m = 100, c/C = dR/1030, C = 0'lamp. 



If, however, we interchange the connexions of the battery 

 and galvanometer, which is simply equivalent to inter- 

 changing the values of n and m in the equation, we obtain 

 for the Anti-Maxwell arrangement : 



Case (B), Anti-Maxwell, 



7i = 100, m=l, c/C = dR/30, C= -002 amp. 



