KXPLUSIONS OF COAL-GAS AND All!. 



377 



liy recording (lie rise of its electrical resistance, which is proportional to its rise 

 of teni|>erature, as it warmed up. The arrangements used to record its rise of 

 resist, mice \\i-ro in principle exactly the same as those used by HOPKINSON for 

 determining (lit- rise <>f resistance of the copper strip in his Recording Calorimeter.* 

 The method consists in passing through the bolometer a constant continuous current 

 sullicient to produce a convenient difference of potential at its terminals, balancing 

 this dillerence by means of a source of constant E.M.F., and recording by means of a 

 mirror galvanometer the rise of potential which occurs when the bolometer gets 

 wanned up. The deflection of the galvanometer is proportional to this rise of 

 potential difference, which, since the current 

 through the bolometer is constant, is pro- 

 portional to the increase of its resistance and, 

 therefore, to its rise of temperature. 



The connections are shown diagrammati- 

 cally in fig. 2. The current C, passing through 

 the bolometer B is taken from a battery of 

 storage cells giving about 100 volts. In this 

 circuit are included a bank of lamps, L, and 

 ammeter, AI. With this arrangement the 

 small variation in the resistance of the bolo- 

 meter as it gets warm has no appreciable 

 effect on Ci since the resistance of the 

 lamps L is very large compared with that 

 of the bolometer. The current C|, which is 

 measured by the ammeter A!, is adjusted to 

 a convenient value before the experiment. 

 The terminals of the bolometer are connected 

 also to the galvanometer G through a resist- 

 ance, R, in which a constant current, C 2 , is 

 maintained by means of two storage cells, (In- 

 direction and magnitude of C 2 being such that 

 the potential difference at the terminals of 

 the bolometer before the experiment is 

 balanced, or approximately balanced. 



The relation between the rise of potential difference at the terminals of. the 

 bolometer and the galvanometer deflection was found by passing small currents 

 through the bolometer, when it was at a known temperature, and noting the 

 galvanometer deflections, C 2 having been reduced to zero during the calibration. 



The recording galvanometer used in these experiments had a resistance of about 

 3'5 ohms. It was of the suspended coil type, with a fairly stiff phosphor-bronze 



Fig. 2. 



NOTE. (', was kept constant during each 

 experiment, but was, in general, different in 

 different experiments. 



VOL. CCXI. A. 



* ' Roy. Soc. Proc.,' A, vol. 79, p. 140. 

 3 C 



