70 Sir W. Thomson on an Electric Machine 



been made and tested for putting in practice the plan of equal- 

 izing potentials, described verbally in the communication to the 

 British Association, which consisted in a mechanical arrangement 

 to produce effects of the same character as those of the water- 

 dropping system, described several years ago at the Royal Insti- 

 tution*. The instrument is represented in the annexed sketch 

 (fig. 3). AT and A' r F are two springs touching a circular row of 



small brass pegsf insulated from one another in a vulcanite disk. 

 These springs are insulated, one or both, and are connected with 

 the two electrodes of the electrometer — or one of them with the 

 insulated part of the electrometer, and the other with the metal 

 enclosing the case, when there is only one insulated electrode. 

 One application is to test the " pyro-electricity " of crystals ; 

 thus a crystal of tourmaline, PN, by means of a metal arm holding 

 its middle, is supported symmetrically with reference to the disk 

 in a position parallel to the line T T', and joining the lines of con- 

 tact of the springs. "When warmed (as is conveniently done by 



* Lecture on Atmospheric Electricity, Proceedings of the Royal Insti- 

 tution, May 1860. See also Nichol's Cyclopeedia, article "Electricity, 

 Atmospheric." 



t [I now find a smaller number of larger disks to be preferable, as consi- 

 derable disturbances are produced by the numerous breakings of contact, 

 unless the two springs are in precisely the same condition as to quality and 

 cleanness of metal surface. Thin stiff platinum pins attached to the disks, 

 and very fine platinum springs touching them as they pass, will probably 

 give good and steady results if the springs are kept very clean. The small- 

 est quantity of the paraffin (with which, as usual in electric instruments, 

 the vulcanite is coated), if getting on either spring, would probably produce 

 immense disturbance. — December 23, 1867.] 



