COMPARISON OF PLATINUM AND GAS THERMOMETERS. 49 



air-tight by suitable protections. This seemingly unimportant detail we found to be 



.1 ijreat a< 1\ :i Hi ;i -,. ;1 > i n i| ,, i|. , i ,, i , -A ..., t !,,!, -\ | -i ],< ,,-,. I during p.:it . !' I I ! .-\ | ..-ri- 



ments, the iiisul.it ic MI of exposed parts always required considerable attention. The 

 lottery was insulated from the wooden case by gutta-percha strip, and after this was 

 added, tin- insulation resistance of the whole apparatus, when all was kept dry, was 



practically perfect. 



XIII. GALVANOMETER. 



For the first experiments the galvanometer employed was one of the pattern 

 descril>ed by DUBOIS and RUBENS in ' Wied. Annalen,' vol. 48, p. 236, lent to us by 

 Professor SCHUSTER. This is a Thomson four-coil instrument with connections so 

 arranged that its bobbins may be coupled to give an internal resistance of 80, 20, or 

 5 ohms. The magnet system and mirror weighed together 0'2 gram. ' The deflections 

 \\ere observed from about three metres distance by a large Steinheil telescope. Much 

 trouble was experienced in finding a foundation for the galvanometer sufficiently free 

 from vibration. After several unsuccessful experiments in which we attempted to 

 insulate the galvanometer with rubber blocks, a special pillar was erected independent 

 of tin floor. We found, however, that, even when resting on this, the vibration of 

 the magnet-system, caused by heavy traffic on the Versailles road, was sufficient at 

 intervals to prevent any satisfactory observations being made. At this juncture 

 I'rolessor CAKI.Y IM>STI-:I; \\as appealed t... and through liini Mr. I!. K. < ii; \ v. of 1 1, ( - 

 India-rubber, Gutta-percha, and Telegraph Works Company, of Silvertown, very 

 kindly came to our aid by sending us a reproduction of an arrangement he had 

 employed at the works to cut off vibrations from delicate instruments. It consisted 

 of a brass plate forming a platform from which the galvanometer is suspended, the 

 whole U-iiiL; sillily liy long india-rubber tubes from a wall-bracket above. To diminish 

 the effect of air currents we added a damping arrangement consisting of a vertical 

 metal cross with attached horizontal vanes, plunging into a vessel standing on the 

 concrete pillar and containing a thick oil. The galvanometer and suspension were 

 also completely surrounded by a paper screen extending upwards to the ceiling, 

 pn>\ ided with suitable openings for making the adjustments. 



The india-rubber suspension arrangement, when once the tubes were properly 

 stretched, worked perfectly satisfactorily till the winter, when, presumably under the 

 influence of the low temperature of about 4 or 5, such a change took place in the 

 elasticity that we were obliged to seek a substitute for the india-rubber, less influenced 

 by climatic, conditions. 



\Ve at length managed to construct from steel wire, 1 millim. in diameter, long 

 spiral springs of the requisite strength, which have served the purpose admirably, and 

 at the same time have shown a comparatively small variation of elasticity with 

 temperature. The arrangement of the . suspension in its modified form is shown in 



s. 5 and G. 



VOL. i \( -IV. A. H 



