340 Messrs. Stewart and Tait on the Heating [June 15, 



3. The deflection of the galvanometer-needle, produced by heating the 

 pile, is recorded by means of a small mirror attached to the needle, which, 

 as it moves, causes the reflected image of a line of light to travel over a 

 graduated scale. The galvanometer-needle is rendered very nearly astatic 

 by means of an auxiliary magnet ; and this arrangement can be made so 

 sensitive that if the temperature of the disk, exposed to the cone as in the 

 figure, were to rise 1 Fahr., this would be denoted by a change in the posi- 

 tion of the line of light equal to fifty divisions of the scale. 



4. In these experiments the disk was rotated rapidly for half a minute, 

 and a heating eifect was, in consequence of this rotation, recorded by the 

 pile. The object of this paper is to investigate the origin of this heating 

 effect. 



5. In this investigation the authors prefer discussing the effect produced 

 on a metallic disk. The metal aluminium was chosen, from its lightness, 

 so as to diminish the weight upon the bearings as much as possible. The 

 reason for preferring a metallic disk is that the heat produced in this case 

 affects the whole substance of the disk, and can thus be approximately 

 measured. The disk of this metal employed was J^th of an inch thick and 

 13 inches in diameter; it weighed ten ounces, and in most of the experi- 

 ments it was covered with a coating of lampblack, applied by means of 

 negative photographic varnish. In some of the experiments a plate of rock- 

 salt, tightly secured in a brass fitting, was screwed upon the mouth of the 

 cone. When this was done, a small piece of anhydrous baryta was placed 

 within the cone to keep the inner surface of the salt dry, and a dish con- 

 taining strong sulphuric acid was likewise placed in the receiver. Indeed 

 the latter was always used ; so that in the results obtained the residual air 

 may be considered as nearly dry, and the surface of the disk, as well as that 

 of the rock-salt, when this was used, nearly free from moisture. 



Furthermore, in order to obviate the objection that the electric currents 

 which take place in a revolving metallic disk might alter the zero of the 

 galvanometer, the position of the line of light was read before the motion 

 began, and immediately after it ceased, the difference being taken to denote 

 the heating effect produced by the rotation. The turning was made in this 

 way: As soon as full speed was obtained, which might be about 10 seconds 

 after beginning the motion, a chronometer was noted, and the handle was 

 turned at a uniform rate for 30 seconds, and thereafter stopped as soon as 

 possible. The most convenient speed in vacua was found to be 20 revolu- 

 tions of the handle, or 2500 of the disk, in 30 seconds. It is believed that 

 the heating effect recorded may be considered as due to about 40 seconds 

 at full speed. 



6. The thermometric value of the indication given by the galvanometer 

 was found in this way : The disk was removed from its attachment and laid 

 upon a mercury-bath of known temperature. It was then attached to its 

 spindle again, being in this position exposed to the pile, and having a 

 temperature higher than that of the pile by a known amount. The deflec- 



