562 Prof. H. L. Callendar on Electrical 



relay is incredibly delicate, and has since been adapted by 

 Brown for the purpose of submarine telegraphy. The con- 

 tacts keep themselves clean automatically, and never require 

 adjustment. Since the deflexion of the galvanometer is 

 limited to a fraction of a millimetre, there is no chance of 

 accidental disturbance of the zero by excessive deflexion. 



The scale of the instrument is independent of the E.M.F. 

 of the battery, being determined by the resistance of the 

 bridge-wire in relation to that of the thermometer. The 

 bridge-wires are usually made with a scale of 2*0, 1*0, or 

 0*5 ohm to a length of 20 cm., which is the length of the 

 record sheet. With a thermometer having a zero resistance 

 of 26 ohms, and a fundamental interval of 10 ohms, these 

 bridge-wires give ranges of 20 c 0., 10° C, and 5° C, respec- 

 tively on the record sheet, or scales of 1, 2, and 4 cm. to 

 1° C. With thermometers of smaller resistances the scales 

 are proportionately smaller and the ranges larger. For 

 clinical work the most convenient range is from 35° to 45° C, 

 with a scale of 2 cm. to 1° 0. This is obtained with a 2G ohm 

 thermometer and a 1 ohm bridge-wire. 



A recorder of this type is more expensive than the thread 

 recorder, but is complete in itself, with the exception of the 

 battery, and does not require either an auxiliary resistance 

 box, or a rheostat for adjusting the scale. It is a great 

 advantage in practice that the scale never requires adjust- 

 ment, but is always correct to about 1 in 1000, provided 

 that the bridge-wire is correct and uniform. It is easy to 

 change from one scale to another when required by changing 

 the bridge-wire, which gives a range of 1 to 4, or by changing 

 the thermometer, which, gives a range of 1 to 20, from a 

 fundamental interval of 10 ohms to a fundamental interval 

 of 0*5 ohm. 



For ordinary work thermometers are generally provided 

 with an u ice-bobbin " or balancing coil, equal in resistance 

 to the thermometer at 0° C. The ice-bobbin for each thermo- 

 meter is connected to its appropriate terminals when the 

 thermometer to which it belongs is in use. If the thermo- 

 meter is required to cover an extensive range of temperature, 

 a series of auxiliary resistances, generally ranging from to 

 20 ohms is provided, which enables the range to be extended 

 to 20 times the range of the 1 ohm bridge-wire. With a 

 26 ohm thermometer the range thus obtained would be 

 200° C., or 2000° C. with a 2*6 ohm pyrometer. For clinical 

 work these auxiliary resistances may be dispensed with if 

 the balancing resistance is adjusted to 30 ohms to balance 

 the thermometer at 40° C, and the zero fixed at the centre 



