46 Scientific Proceedings, Royal Dublin Society. 



The results given above show that the initial amplitude has a verj- 

 distinct influence on the rate of subsidence of torsional oscillations : the greater 

 the initial amplitude the lower is the longitudinal magnetic field in which 

 the maximum internal friction in the wire takes place. The results obtained 

 with the small initial amplitude of about 7° will bear more directly on any 

 future explanation of the phenomena of magnetisation and torsion that we 

 may attempt, because the angular displacement of the molecules of the wire 

 is about tlie same both in the torsion and the subsidence experiments. 



Tliat the main origin of the damping of the oscillations must exist inside 

 the material of the wire, is, I tliiuk, shown by the following experiments. 

 The mercury and the iron pin dipping into the mercury at the lower end of 

 the vibrator were thoroughly cleaned and arranged so that the pin dipped into 

 the mercury about one millimetre only, so as to reduce the mechanical friction 

 to a minimum ; the longitudinal load on the wire was the same as before, 

 namely, 3 x 10° grammes per square centimetre, and a longitudinal magnetic 

 field of 5 units was put round the wire. The following five sets of observa- 

 tions were made on the rate of subsidence of tlie torsional oscillations, each 

 starting from an initial amplitude of 90° : (1) when the wire was oscillating 

 quite freely ; (2) when the iron pin was dipping into the mercury, but the 

 circuit open; (3) when the oscillating wire was short-circuited ; (4) when the 

 wire circuit was closed through an inductive coil of 10,000 ohins resistance ; 

 (5) when the circuit was closed through a condenser of i microfarad capacity. 

 In each of the five cases the resultant curves were identical. 



Section 4. — Influence of Diameter of Wire. 



It has been shown that when the longitudinal load per unit cross- 

 sectional area on a nickel wire is constant, the longitudinal magnetic field in 

 which the maximum twist of the free end takes place is independent of the 

 cross-sectional area, and that the larger twist is obtained with the larger 

 wire.^ Experiments wei-e therefore made on the subsidence of torsional 

 oscillations, with nickel wires of different diameters, and in order to have 

 them as nearly as possible in the same ])liysical condition, they were tested in 

 the state in which they were received from the manufacturer, who subjected 

 all the wires to the same heat-treatment. 



Six different sizes of nickel wires were used — Nos. 20 to 14 S. \Y. G-., 

 inclusive — and were found to be very hard, the simple rigidity being 770 x 10'^ 

 grammes per square centimetre ; they were 226 centimetres long, and wlien 

 under test each was loaded at the rate of 2 x 10* grammes per square 



' Sclent. Prof. Roy. Dub. Soc, vol. xii.. No. 37, p. 613. 



