Mr. C. Tomlin son's Experiments on the Electrical Fly. 209 



seeking to discharge upon the prime conductor, but not being 

 able to do so, there can be no motion of the fly*. 



3. The Fly enclosed. 



When the insulated fly is covered with a bell-glass and elec- 

 trified, there can be no continuous motion, because the particles 

 of air cannot discharge themselves and escape to make room for 

 a continuous succession of uncharged particles from a distance. 

 CavahVs experiment was repeated in the following form : — A 

 copper disc 6 inches in diameter was placed on an insulating 

 stand, and the fly on the centre of the disc (see fig. 2) . The 

 fly was now covered with a clean dry bell-glass receiver 12 inches 

 high and 4^ inches in diameter; and a chain was carried from 

 the prime conductor into contact with the copper disc. On 

 working the machine, the fly rotated a few times and then 

 stopped. On touching the stand there was no motion of the 

 fly. On touching the copper disc, the fly made a few turns and 

 then stopped. On presenting the finger to the outside of the 

 glass on a level with the fiy at the distance of 8 or 10 inches, 

 the fly immediately started off* at a rapid pace, and continued to 

 rotate so long as the finger was held. When the motion of the 

 fly ceased, a sharp metal point was presented at the distance of 

 27 inches, and the fly started off again. 



Cavallo explains this effect with considerable sagacity. The 

 mistake made by him, in common with some more modern 

 writers, seems to be in describing the electricity as passing off so 

 readily from a point and so electrifying the air ; whereas it is 

 the air that fetches the electricity from the point and so dissipates 

 the charge. In the experiment just narrated the electricity is 

 conveyed into the bell-glass by the fly, and its metal support and 

 the confined portion of air are made polar, but cannot discharge 

 upon the points because, being confined and insulated, there can 

 be no circulation. For a similar reason, it cannot get rid of its 

 surplus electricity by charging the bell-glass and returning to 

 the points for a fresh charge. Directly, however, a conducting 

 point is presented to the outside of the bell-glass, a portion of 

 the vitreous electricity is drawn off from the outside, the inside 

 becomes capable of receiving a charge, and the particles of air 



* I need scarcely in this place answer the question which has been fre- 

 quently put to me in the course of these experiments, why the fly insulated 

 on the prime conductor rotates, when if insulated apart from, but near to, 

 the prime conductor it does not rotate. It is obvious that on the prime 

 conductor the particles of electrified air can discharge themselves upon the 

 points of the fly, while insulated away from the prime conductor it forms 

 part of the dielectric medium ; uninsulated away from the prime conductor, 

 the charged particles of air can discharge themselves upon the points of 

 the flv and thus produce motion. 



Phil. Mag. S. 4. Vol. 27. No. 181. March 186 k P 



