Recording Thermometers for Clinical Work. 545 



due to defects of insulation or imperfect contacts, or small 

 differences in the resistance of the thermometer and com- 

 pensator leads. On the other hand, for practical applications,. 

 or for ink recording, it is necessary to employ a fairly robust 

 and portable galvanometer of the suspended or pivotted coil 

 type ; the current employed must be considerably increased 

 if a large scale record is required, and it may be important 

 to choose the resistance of the thermometer to suit the- 

 galvanometer or vice versa, in order to minimise the heating 

 effect of the current. 



The symmetry- of the above expression in respect of G 

 and R shows that the condition governing the choice of the 

 resistance of the thermometer, if the galvanometer is given 

 and m is constant, is the same as that already given for the 

 galvanometer in terms of R, namely, 



R=2G/(l+m). 



If, however, the value of mR = S is constant, being fixed 

 by the resistance of the ratio coils, each equal to S, of the 

 bridge employed, the maximum sensitiveness is obtained, 

 when 



R = 2G + S. 



The latter condition gives the limit beyond which no^ ad- 

 vantage can be gained by increasing R and diminishing m, 

 even if the heating effect of the current on S can be entirely 

 neglected. If the heating effect of the current in the 

 thermometer is kept constant, as already assumed, the 

 heating effect in S increases in proportion "to R/S. On the 

 same assumption the total current to be supplied bv the 

 battery (which is a consideration in records of long duration) 

 is a minimum when R=S, but is only increased in the ratio 

 5/4 when R = 4S. The maximum sensitiveness is obtained 

 when S = and R=2G, but the sensitiveness will be reduced 

 by little more than 10 per cent, if we make S = G/2, keeping 

 R = 2G. It would be reduced a further 20 per cent, if we 

 made R = G = S, according to Maxwell's investigation *. 



In employing the deflexion method described below, the 

 advantage of making R larger and S smaller than G is 

 really greater than would appear at first sight, because, if 

 the sensitiveness is reduced 20 per cent, for the same heating 

 effect of the current, it means in practice that the current 

 must be increased 20 per cent, to give the required deflexion, 

 * Maxwell, <■ Electricity and Magnetism,' vol. i. §349. This is the 

 right arrangement if all "the four arms of the bridge are similar, and 

 equally affected by the current. 



Phil. Mag. S. 6. Vol. 19. No. 112. April 1910. 2 N 



