Ferromagnetic Wires by Magnetization. 341 



made to touch the vertically suspended wire with suitable 

 pressure, and a small rotation of the mirror given at our 

 disposal. This arrangement is omitted in the above figure-. 

 The rotating cylinder is made of steel and is 1*51 mm. thick, 

 and the sensibility of our apparatus is such that one division 

 of the micrometer ocular corresponds to a change of length 

 of 2' 12 x 10 -7 per cm. of the ferromagnetic wire. 



3. The method of observation was as follows : — The wire 

 to be tested was hung vertically and stretched bv a weight 

 of 5 kilograms for three or four hours to make it straight. 

 To begin with, all the weights were removed, and then the 

 wire was again loaded with -J or 1 kilogram. The wire was 

 then completely demagnetized, and magnetized by passing 

 gradually increasing currents, and the corresponding de- 

 flexions were taken, the demagnetization being effected before 

 each magnetization. A set of observations being thus taken, 

 successively increasing loadings were applied and the cor- 

 responding sets of observations noted. 



Since the resistance of the magnetizing coil was onlv 

 0*6 O the thermal expansion of the ferromagnetic wire due 

 to the magnetizing current was negligibly small for the 

 currents used in the present experiments. The electromagnetic 

 action between the steel cylinder and the coil was also found 

 to be negligibly small. When the pressure in the contact 

 surface between the cylinder and the wire was moderate 

 repeated applications and removals of the magnetizing field 

 showed no trace of slipping on the cylinder. 



The wires tested had the following dimensions : — 



Meta ] s Soft iron. Wolfram steel. Nickel. Nickel-steel Nickel-steel. 



(45 p. c. Ni). (35 p. c. Ni). 



Length ... 2074 cm. 20-97 cm. 20-70 cm. 2075 cm. 2073 cm. 

 Diameter... 0439 0-060 0136 0-150 0144 



4. Soft Iron. — Fig. 4 (p. 342) represents the curves of the 

 change of length plotted against the magnetizing field ; T i> the 

 ten-ion per square millimetre. The specimen was very well 

 annealed, and so the initial elongation was greatly reduced. 

 From the figure we see that the effect of tension is to reduce 

 the initial elongation. This reduction becomes greater as 

 the tension increases, till the initial elongation Aanishes at a 

 tension of about 4" kilograms per square millimetre. When 

 the tension exceeds this value the curvature of the curve is 

 reversed. In fields greater than 40 c.G.s. units all the curves 

 are nearly parallel to each other. The effect of tension on 

 the change of length is comparatively larger when the load is 

 small than when it is heavy. 



