122 Scientific Proceedings, Royal Dublin Society. 



were assembled, that is, four wires grouped round one in the centre of the 

 tube, they were firmly bound together and given a coating of shellac varnish. 

 The assembled wires were then firmly fixed in the tube, and the circuit 

 arranged so that the electric current went through the five wires in the same 

 direction, producing a transverse magnetic field in the gap or slot. The 

 strength of the magnetic field in the gap was proportional to the total 

 current through the wires, and, by plotting the values of the current as 

 abscissse and the corresponding values of the field as ordinates, if a straight 

 line be drawn from the origin to the point corresponding to current 

 = 360 amperes (72 amperes in each wire), and magnetic field = 1000 units, 

 all the intermediate points will lie on that line. The tube, with the wires 

 inside, was fixed vertically against the wall with adjustable supports, and the 

 nickel wire under test was suspended in the middle of the gap from a separate 

 support. The nickel wire was kept in the middle of the gap by means of thin 

 strips of wood along each side throughout its whole length, so that, when the 

 transverse fields were applied there was no motion of the wire to either face of 

 the slot. The total length of the nickel wire was 225 cms., and the length 

 of the iron tube 215 cms., thus leaving 5 cms. of the nickel wire at top and 

 bottom free from the influence of the field in the gap of the iron tube. 



The load on the lower end of the wire was applied by means of a brass 

 cylinder 1'2 cms. diameter, having at one end a three-jaw clutch for attaching 

 to the wire, and at the other a hook to which a cord was fixed, and then passed 

 over two frictionless pulleys to a scale-pan for holding the required weights. 



On account of the great length of the wire under test, the change in the 

 length on the application of the magnetic fields could be read off directly by 

 means of a microscope having a fine hair in the eye-piece, one of a number of 

 fine lines on the brass cylinder serving as an index mark. The movement 

 of the hair was made by means of a micrometer screw with graduated head 

 or cap, one division of which was equivalent to a change of 9*2 x 10- 7 per 

 unit length of the wire. 



The nickel wire tested was 0T69 cm. in diameter, having rigidity 

 = 708 x 10 6 grammes per sq. cm., and was loaded with a weight equivalent to 

 2 x 10 5 grammes per sq. cm. The wire was subjected to direct transverse 

 magnetic fields up to a maximum value of 1000 units, and to alternating 

 transverse magnetic fields of maximum value 200 units ; but, for the sake of 

 comparison with previous work with longitudinal fields, the values of the 

 expansion here given in the table lie within a maximum of 200 units for both 

 direct and alternating fields. 



In the table the values in column marked H are the strengths of the 

 applied magnetic fields, D.O. representing direct or continuous fields, and 



