210 MR. S. BIDWELL ON CHANGES PRODUCED BY MAGNETISATION IN 



illuminated by a lime light, the image of a horizontal wire is, after reflection from the 

 mirror, projected upon a distant vertical scale. A very slight deflection of the mirror 

 causes a considerable movement of the image. The actual dimensions are as follows : 

 -The distance AB =10 mm., AC =170 mm., MC = 7 mm. ; the distance from the 

 mirror to the scale was in all the experiments described in the present paper 7320 mm. 

 (24 feet). The multiplying power therefore (remembering that the angular deflection 

 of the beam of light is twice that of the mirror) was 



7320 x 170 x 2 



7 x 1Q = 35,554 times. 



Each division of the scale = 4^ inch, = '04 mm. Therefore a movement through one 

 scale division indicated a difference in the diameter of the ring, or the length of rod, 

 under examination of 



1/35554 of 0-64 mm. = O'OOOOIS mm.* 



Since the mean diameter of the rings is 60 mm., one scale division corresponds to 



0-000018/60 = 0-0000003 of diameter, 



or three ten-millionths. 



The rods being 100 mm. long, one scale division indicates 0*00000018, or l - 8 

 ten-millionths of their length. 



The optical arrangements were very perfect, and it was easy to read to half a scale 

 division, or less, with accuracy. 



The currents were measured by one of AYRTON and PERRY'S ammeters, having a 

 commutator by means of which the coils could be arranged either in series or in 

 parallel, the deflections in the former case being ten times as great as in the latter. 

 The instrument had been recently calibrated and was checked by reference to a tangent 

 galvanometer. The ammeter being nearly dead-beat, the circuit needed to be closed 

 only for a very short time in taking a reading. But it was found necessary in the 

 experiments with the rings to switch the current through a coil of the same resistance 

 before measuring it. Th3 coils employed for this purpose were wound with the same 

 kind of wire as that used for the rings. The ammeter was read to a quarter of a scale 

 division. 



The experiments were conducted as follows : 



(a) The ring or rod was demagnetised. 



(b) A certain current was passed for a moment through the magnetising coil and 



the deflection indicating elongation or retraction was noted. 



(c) The same current was passed a second time and the deflection noted. This was 



generally different from that given by (6). 



(d) The iron was once more demagnetised. 



* Seven ten-milliontbs of an inch. 



