204 



ME. W. F. G. SWANN ON THE SPECIFIC HEATS OF AIR AND 



Tig A. 



of thick platinum wire, which was fused into two thin glass tubes, one surrounding each 

 of the leads. The glass tubes were brought side by side, and made to grip one of 

 the mica frames. A length of about 100 cm. of '008-inch pure platinum wire was 

 bent double, and the loop end was gold-soldered to the junction of a current and a 

 potential lead. The two strands were wound in parallel on the mica frame, the free 

 ends being gold-soldered to a piece of thick platinum wire, which was passed up 

 the frame, and was soldered to the junction of the other current and potential leads. 

 The resistance of the coil so constructed was just over 1 ohm at the temperature of 

 the room. A second mica frame was slid up between the glass tubes until its centre 

 was about 8 cm. from that of the heating coil, and one of the platinum thermometers 

 was wound on this frame from "004-inch pure platinum wire, the ends of its 

 compensating leads being joined by a piece of the same platinum wire in the manner 

 adopted by Prof. CALLENDAR for the compensation of the cooling effect of the 

 leads on the thin thermometer wire. To insure insulation from the glass surfaces 

 surrounding the heating coil leads, the thermometer leads were enclosed in fine glass 

 tubes, the whole being bound firmly together with silk. 



The thermometer above referred to served to measure the temperatm-e of the gas 

 after it had been heated by the electric current ; it will be referred to as the ther- 

 mometer H. The other thermometer K was wound in a 

 similar manner. Several little discs of copper gauze 

 were threaded between the heating coil and the ther- 

 mometer H. These discs, in virtue of their high thermal 

 conductivity, served to distribute the heat uniformly 

 throughout the gas as it passed from the heating coil to 

 the thermometer. The external thermometer leads were 

 soldered to the internal leads, and were bound with 

 insulating binding to ebonite heads, fig. 4, fixed firmly 

 on the glass tubes which projected from the ends of the 

 thermometers. The method adopted for connecting the 

 internal and external current and potential leads is 

 indicated in fig. 4. The four leads coming up through 

 the hole in the ebonite head E are bent over, and dip into 

 four mercury cups in an ebonite horse-shoe, which was 

 supported from a clamp by the handle L ; the external 

 leads were brought to these mercury cups. This arrange- 

 ment enabled the connections to the internal current and potential leads to be 

 interchanged, to eliminate possible difference of exact similarity between them. 



(6) The Platinum Thermometer Bridge and the Measurement of the Fundamental 

 Intervals. The bridge was of the well-known form designed by Prof. CALLENDAR, 

 with mercury cups in place of plugs. Two calibrations of the coils and of the bridge 

 wire were made, full details of which have been preserved. No two corresponding 



