Recording Thermometers for Clinical Work. 561 



heating effect were found to be a tenth of: a degree with the 

 galvanometer adjusted for a scale of 2 cm. to 1° C, it would 

 be four tenths if the scale were increased to 4 cm. per 

 degree, and the zero would be raised three tenths. 



Calibrat'on of the Scale of the Galcanometer. 



The angular deflexion of the galvanometer for one milli- 

 volt is rather large, and the scale may not be accurately one 

 of equal parts. This should be tested for accurate work. 

 With the aid of the bridge it is easy to make the test, either 

 by placing the thermometer in water at a steady temperature, 

 or by substituting a fixed resistance, equal to the balancing 

 coil, in place of the thermometer. Balance the galvanometer 

 at zero by adjusting the bridge-wire contact. Adjust the 

 scale to the required value. Unplug successively resistances 

 in R corresponding to 1, 2, 3, 4, 5°, and observe the galvano- 

 meter reading for each on the record sheet. Repeat in 

 descending order to test the galvanometer for change of zero 

 due to imperfect elasticity of the suspension. The errors 

 should not as a rule be appreciable on the scale of the record 

 unless there is something wrong with the levelling or sus- 

 pension. It is of course extremely important that the 

 galvanometer coil should be perfectly free in all positions of 

 the boom, and it must be remembered that a variation of 

 level may cause a variation of scale, as well as of zero. 



Balance Method with Slide-wire Recorder. 



10. This is the type of recorder generally employed with 

 resistance thermometers, especially where a large range of 

 temperature is to be covered. It is equally applicable for 

 clinical work, or for small ranges of temperature, provided 

 that a suitable thermometer is employed, and that the current 

 is properly adjusted to avoid excessive heating of the thermo- 

 meter. It gives a strictly continuous ink record, with a pen 

 attached to the sliding contact on the bridge-wire, which is 

 automatically maintained at the balance point by means of 

 a pair of motor clocks actuated by the deflexion of the 

 galvanometer. The galvanometer coil carries a light arm 

 terminating in a contact fork consisting of two short platinum 

 wires close together. A wheel with a platinum edge rotates 

 between these two wires, one or other of which makes 

 contact with the wheel (according to the direction of de- 

 flexion of the galvanometer), and completes a relay circuit 

 actuating one or other of the motor clocks. This form of 



Phil Mag. S. 6. Vol. 19. No. 112. April 1010. 2 



