Electrical Discharge. 343 



external air, as in fig. 4 ; and as the whole was sustained upon 

 a horizontal plane of wood, having free motion upon a similar 

 plane beneath, either extremity of the instrument could be ele- 

 vated within certain limits, so as to give the tube of the ther- 

 mometer a greater or less degree of inclination. This is, in fact, 

 the form of my instrument resorted to by M. Riess, and figured 

 by M. De la Rive in his recent work, Traife de I' Electricite, 

 vol. ii. p. 156; as also by M. Pouillet, Elemens de Physique, 

 who refers the instrument to M. Knockenhauer. 



In the course of my experiments with this form of the instru- 

 ment, I sometimes employed long and delicate tubes containing a 

 very small hne of coloured fluid, moveable within them as an index 

 merely. I also occasionally used a small column of mercury in 

 the same way. After all my efforts, however, to perfect the instru- 

 ment, I found no form so really efiicient and simple as that of 

 the instrument shown in fig. 5. In this figure, pvnd is the ther- 

 mometer ball as before, capped and screwed at d upon a reser- 

 voir of coloured fluid, do, and from which the thermometer tube 

 is extended. The tube cab is twice bent, so as to bring it into 

 a vertical position, ab, parallel with the reservoir, cd; the whole 

 is mounted upon a small elliptical base of wood, sustained on 

 three or four screw feet, as shown in the figure ; the vertical 

 portion of the tube, ab, being secured to, and sustained by, a 

 rigid divided scale fixed to a support of wood springing out of 

 the elliptical base. 



The point o of the level of the liquid in the tube is marked 

 zero on the scale. \ATien an electrical discharge of a given force 

 traverses the wire p n, the fluid is observed to ascend along the 

 scale, indicating the comparative degree of heat excited in the 

 wire. It is to be observed that the diameter of the reservoir, cd, 

 which is a sort of hollow flattened ball, is sufficiently great to 

 render the difference of the level of the fluid in the reservoir, 

 caused by the abstraction of the quantity which ascends in the 

 tube along the scale, infinitely small. 



This form of the instrument is figured in the Transactions of 

 the Royal Society for 1827, in the Memoirs of the Plymouth 

 Institution for 1830, and also in the Transactions of the Royal 

 Society of Edinburgh for 1834*, where it will be found applied 

 tt^ the purposes of voltaic electricity. When we employ a long 

 iioe wire of platinum in the ball turned into a helix, the instru- 

 ment becomes marvellously sensitive. In order to facilitate ex- 

 periments with different metals, I have sometimes employed a 

 ball pierced in many points of its circumference for the reception 

 of several wires, as shown in fig. G. I also employed a cylin- 

 drical bulb, shown in fig. 7, in which thewircs could be either fixed 

 * Vol. xii. 



