100 Scientific Proceedings, Royal Dublin Society. 



gap or slot, and the strength of the current through the central copper wire, 

 was found in the usual way, by means of an exploring coil, earth inductor, 

 and ballistic galvanometer. The galvanometer employed was a moving-coil 

 bifilar suspension ballistic galvanometer with a period of eleven seconds. 



The strength of the magnetic field in the gap was found to be proportional 

 to the current strength in the copper wire, in such a way that if we plot 

 as abscissae the values of the current in the copper wire and as ordinates 

 the corresponding values of the magnetic field strength in the slot, the points 

 will lie on a straight line passing through the origin. Thus, if a line be 

 drawn from the origin to a point corresponding to current = 80 amperes and 

 magnetic field = 300 units, all the intermediate values will lie on that 

 line. 



The iron tube and copper wire were fixed vertically against the wall with 

 suitable adjusting supports, and the nickel wire to be tested was suspended 

 separately so as to hang in the middle of the slot in the iron tube. The free 

 length of the nickel wire was 225 cms., thus leaving 2 - 5 cms. free from the iron 

 tube at the top and bottom, the vibrating load on the lower end of the nickel 

 wire had a small mirror attached, the distance of this mirror from the scale 

 on which the amplitudes of the torsional oscillations were read, by means of 

 a light-spot, was 167 cms. The maximum amplitude used was at the mark 

 300 on the millimetre scale, which corresponded to an angular twist of the 

 lower end of the wire of about 5° 10' on each side of the zero on the 

 scale. 



The vibrating load was a non-magnetic cylinder composed of lead and 

 brass, and the torsional oscillations were set up by the application of two 

 properly timed simultaneous puffs of air which strike tangentially on the sides 

 of the vibrator. A great many preliminary experiments were performed in 

 order to find out the best method of procedure and also the most suitable load 

 to use on the wire. "With a given load on the wire, the subsidence of torsional 

 oscillation was observed when no transverse magnetic field was on the wire, 

 that is, when the wire was under the influence of the field due to the earth's 

 magnetic force only; then a given electric current was sent through the central 

 copper wire which gave a transverse magnetic field in the gap or through the 

 nickel wire, and a series of observations of the torsional oscillations were 

 again taken. This was done for magnetic fields of various values both direct 

 and alternating. It was found that when the strength of the magnetic field 

 approached 120 units, the nickel wire was inclined to be attracted to one 

 or other face of the slot. In order to get over this tendency of the wire to 

 move towards the side of the slot, the following plan was adopted : — One end 

 of a string of torsionless floss silk was fixed by means of a small three -jaw 



