212 Mr. C. Tomlinson's Experiments on the Electrical Fly. 



uninsulating the apparatus by touching the air-pump table. 

 When the room was darkened, brushes were seen from the four 

 points of the fly. 



Here I imagine the dielectric particles were not in sufficient 

 number to produce mechanical action by their discharge upon 

 the points. When the gauge stood at 2 inches, the fly spun 

 round with great rapidity on presenting the finger to the receiver ; 

 and its sensibility went on increasing as more air was admitted, 

 so that at 5^ inches' pressure the fly obeyed the action of a metal 

 point held 14 inches and upwards from the receiver. When the 

 mercury stood at 8 inches, the fly was very sensitive and revolved 

 rapidly with a librating motion towards the finger. If the ope- 

 rator stood on an electrical stool and presented a finger or metal 

 point to the receiver, there was no motion even when it was most 

 sensitive. 



Here the glass became converted into a Leyden jar, the inside 

 highly charged with vitreous electricity, and the outside with 

 resinous, brushes of which were thrown off freely on presenting 

 the finger, which served as a vent to the outside ; while the air 

 on the inside, by constantly discharging on the points, thus got 

 rid of the surplus vitreous charge as fast as it was poured in. 



5. The Fly in liquid dielectrics. 



Faraday found* that when a dielectric, such as turpentine, 

 was electrified in a glass vessel by means of two wires dipping 

 into it in different places, "one leading to the electrical machine 

 and the other to the discharging train, on working the machine 

 the fluid will be thrown into violent motion throughout its whole 

 mass, whilst at the same time it will rise two, three, or four 

 inches up the machine wire and dart off in jets from it into the 

 air." 



In the museum of this College is an apparatus fitted up by my 

 friend the late Professor Daniell for showing these effects. It 

 consists of a finger-basin with a wire passing through the bottom 

 for the purpose of mounting on the prime conductor. Turpen- 

 tine containing particles of gold-leaf was poured into the vessel, 

 and on working the electrical machine and holding a metal knob 

 over the vessel, the currents in the dielectric were made visible by 

 means of the gold-leaf particles. I tried to mount the fly in this 

 apparatus, but not succeeding to my mind, I drilled a hole in 

 the bottom of a finger-glass, passed a cork into the hole, and 

 ran a darning-needle through the cork so as to project into the 

 vessel more or less as required. The glass was mounted on an 

 insulating stand, and a chain from the exterior portion of the 

 needle led to the prime conductor ; but still further to insure 

 * Experimental Researches in Electricity, § 1595. 



