164 



Scientific Proceedings, Royal Dublin Society. 



total current through the wires as abscissae and the corresponding values of 

 the magnetic field as ordinates, the points were found to lie on a straight line 

 passing through the origin ; thus, if a straight line be drawn from the origin 

 to the point corresponding to current = 200 amperes (40 amperes in each 

 wire) and magnetic field = 90 c.g.s. units, all the intermediate points will lie 

 on that line. 



Fig. 1. 



S is a solenoid 236 cms. long, consisting of 7707 turns in four layers, and 

 giving a longitudinal magnetic field of 41 c.g.s. units per ampere, this field 

 being uniform throughout the length of the coil to within 5 ems. of each 

 end. T is the wire under test, and B is one of the two hinged brass clamps 

 which support the apparatus on a strong vertical piece of hard wood, that can 

 be adjusted in two directions by means of suitable screws. The wire T is 

 suspended from an independent support, and hangs vertically in the middle 

 of the solenoid S. 



To the lower end of the wire is fixed a non -magnetic load, and from a 

 plane mirror attached to this end, a beam of light is thrown on to a millimetre 

 scale placed at a distance of 167 cms. and by this means the twist of the 

 lower end of the wire may be read to within 0'2 mm. on the scale. On 



