176 



Scientific Proceedings, Royal Dublin Society. 



uniform strength, by means of two three-]" caw clutches at a, a, fig. 1. These 

 clutches were made in the ends of brass rods, half an inch in diameter, and 

 gripped the wire firmly; the top rod was fixed to a heavy wooden beam in 

 the ceiling of the room, and to the lower rod a brass vibrator was fixed, the 

 weight of whicli could be altered by means of discs of lead. From tlie lower 

 end of tliis vibrator an iron wire dipped into a meroary-cup, so as to complete 

 the circuit through the galvanometer. Tlie brass rods and clutches were 

 both covered with rubber-tubing insulation, so as to prevent them from coming 

 in contact with tlie inner tube of the solenoid, and tliereby avoid any leakage 



'^^mm0wwwww 



of the transitory electric current, which is produced when the lower end of 

 the iron wire is twisted. A secondary battery, an ammeter, a reversing kej', 

 and suitable rheostat, were put in series with the solenoid, so as to produce 

 the desired longitudinal magnetic field round the wire imder test. 



That the transitory electric current produced when an iron wire is twisted 

 in a magnetic field is entirely due to the j^resence of the field was shown by 

 the following experiment :— An annealed, No. 16 iron wire, being suspended 



