Recording Thermometers for Clinical Work. 543 



having a resistance about 25*6 ohms at 0° C, with a funda- 

 mental interval of 10 ohms, and have made the ratio coils 

 ^bout 6*4 ohms, giving m = l/4*, which gives an advantage 

 of about 20 per cent, in point of sensitiveness as compared 

 with m = l. A more important consideration in this case is 

 that the same box can be used with thermometers of lower 

 resistance, e.g. R =2'56 ohms, F.I.=1 ohm, without serious 

 loss of sensitiveness. 



Resistance of Galvanometer. 



4. It is generally possible to vary the resistance of a 

 .galvanometer through a wide range, by altering the con- 

 nexions of the coils or by substituting one coil for another, 

 without altering the mass of the coils. In this case the 

 sensitiveness will vary approximately as the square root of 

 the resistance Gr. We see from equation (1) that the de- 

 flexion of the galvanometer, which is proportional to c^/Gr, 

 will be a maximum when 



G=n(l + m)R'/(l + n). 



This result agrees with that given by Maxwell, and shows 

 that in the majority of cases which occur in practice, G 

 should be of the same order of magnitude as R, being 

 restricted to the limits 2R and R/2, if n is never less than 1, 

 or m greater than 1. If we are restricted, as in platinum 

 thermometry, to the case n = l, we should take 



G=(l + m)R'/2.' 



The best resistance for the galvanometer is restricted to the 

 same limits, 2R and R/2, provided that m does not exceed 3. 

 For example, in the case of the box already quoted, with 

 ratio coils 6*4 ohms, when used with a thermometer R =25*6 

 ohms, the best resistance for the galvanometer was 16 ohms. 

 When the same box was used with a thermometer ten times 



* The reason for choosing this particular value of m in the case of 

 the compensated box with platinum-silver and platinum coils, which 

 was exhibited at the Royal Society in May, 1893, and fully described 

 later (Phil. Trans. R. S. A., vol. 199, p. 92), was that the change of 

 resistance of the platinum-silver coils due to the heating effect ot the 

 ■current might be of the same order of magnitude as that of the platinum 

 coils, and might take effect at the same rate. The thermal capacities 

 and radiating surfaces of the coils being nearly equal, but the temper- 

 ature coefficient of the platinum-silver nearly 1*6 times less than that of 

 the platinum, the platinum-silver coils would carry about 4 times as 

 great a current as the platinum for the same change of resistance. This 

 was found to give a fairly safe limit for the value of m, but the effect 

 was so small as to be of little importance. 



