88 Prof. Magnus on the Propagation of Heat in Gases. 



by means of a very delicate multiplier with a double needle, 

 which was connected by copper wires with the binding screws 

 L L. The wire of the multiplier consisted of copper which had 

 been galvanoplastically deposited, and was free from iron. It is 

 the multiplier which I used in my investigation on thermo-electric 

 currents. I tried to use in its stead a multiplier with a steel 

 mirror, which was read by means of a telescope and scale ; but 

 spite of this mode of observation I found it less delicate, mani- 

 festly because the steel mirror was not astatic. Whether a 

 multiplier with mirror and astatic needle would not be better 

 for these observations I have not tried; but I doubt it ; for the 

 reading off by means of a mirror is only suited for small dif- 

 ferences of angles, while in using an astatic needle greater 

 deviations are observed. The multiplier used was placed upon a 

 firm stand separate from the rest of the apparatus. 



In investigating the diathermancy of a gas, water at 100° C. 

 was poured into the vessel C, and kept at that temperature by 

 means of steam passed into it from a flask in the neighbourhood. 

 The moment the screen ccee was displaced, the needle began 

 to move slowly, and after it had reached its greatest deflection, 

 it assumed a fixed position in the course of about two minutes, 

 after a few very small oscillations. This was read off partly 

 directly, and partly by means of a telescope ; in the latter case 

 a rectangular prism was placed directly over that part of the 

 divided circle which was to be obseiwed, so that the position of 

 the needle could be seen by reflexion. When this was effected, 

 the screen was replaced over the pile, upon which the needle 

 reverted to its original position. It was never, however, exactly 

 over the null-point of the scale, either because the torsion of the 

 wire had changed a little, or because there had been a slight 

 difference of temperature in the pile. As in the multiplier used 

 the coils of wire were quite free from magnetism, the replace- 

 ment at 0° could easily have been effected by turning the 

 divided circle independently of the magnetic needle. But this 

 might easily have produced fluctuations of the entire apparatus, 

 and it therefore appeared better to take the mean of the positions 

 of. equilibrium before and after the deflection, and to subtract 

 this from the observed deflection. The observations thus ob- 

 tained agreed very well with each other when several were made 

 successively. It was only after the experiments had been con- 

 tinued some time that the numbers somewhat disagreed, because 

 the pile became a little warmer at one end. Four to six of such 

 observations were always made in succession, and the mean of 

 these taken. 



The values corresponding to the deflections of the galvano- 

 meter were determined by the method given by Melloni in his 





