182 Scientific Proceedings^ Royal Dublin Society. 



that as the cross-sectional area of the wire is increased about 4'7 times, the 

 longitudinal magnetic field required to give the liigliest value of the maximum 

 transitory current is decreased from 3-2 to 2-3 c. g. s. units, or about 28 per cent. 

 Again, if the highest values of the maximum transitory current be picked off 

 from the first four curves in fig. 3, and plotted as ordinates against the cross- 

 sectional area of the wires as abscisste, the four points will be found to lie 

 practically in a straight line. This shows that as the cross-sectional area of 

 the wire is increased 4-7 times, the highest value of the maximum ti-ansitory 

 current increases from 56 to 207 galvanometer scale-divisions, or about 

 3'7 times. 



Tlie arrangement of the apparatus was now changed so as to obtain the 

 circular magnetisation, and to measure the amount of twist in an iron wire 

 when carrying an electric current and when placed in a uniform longitudinal 

 magnetic field throughout its entire length. 



The vibrator or weight at the end of the wire was modified in such a 

 way as to allow the reflecting surface of a small concave mirror to be coin- 

 cident with the axis of the wire under test, the wire being at the same time 

 in a uniform magnetic field. The stem of the vibrator which was used in the 

 experiments just mentioned was cut, and a brass tube 6 cms. long and 3 cms. 

 diameter inserted, in which were arranged the mirror and its means of adjust- 

 ment as sliown in fig. 4. At tlie upper end a is a three-jaw clutch for 

 clamping the wire inside the solenoid, and at 6 is a torsion-head with some- 

 what stiff motion, by means of which the lower part of the vibrator can be 

 turned round the axis of the wire with respect to the upper part a b held 

 stationary. By this means the spot of light from the small mirror m could 

 be brought on to the scale when the wire under test was in position inside 

 the solenoid. 



The mirror m is fixed on a small hinged plane surface which can be given 

 two motions, viz., a motion at right angles to the vertical wire by means of a 

 small slide-rest and the screw d, and a very slight motion round the hinge or 

 pivot of tiie mirror by means of the screw c. The first movement brings the 

 reflecting surface of the mirror in the vertical plane through the axis of the 

 wire, and the second gives a means of getting the spot of light on to the 

 scale. The scale used was divided into 700 divisions in a length of 44 cms., 

 and was placed at a distance of 109 cms. from the mirror. 



In the following tests the longitudinal magnetisation round the wire was 

 obtained by means of the long solenoid, and the circular magnetization 

 by sending a certain definite electric current through the wire; each of 

 these circuits had its own independent accessories in the form of secondary 

 battery, variable resistance, ammeter, reversing key, and plug-key. The 



