1902.] Longitudinal Magnetic Field on Internal Viscosity. 29 7 



rod B about a metre long and 5 mm. in diameter. To the lower end 

 of this glass rod is rigidly fastened a cylindrical vibrator C. The axes 

 of the wire, the glass rod, and the vibrator are as nearly as possible in 

 one vertical straight line, and the three bodies are rigidly connected, 

 so that if the vibrator is turned round its axis, no slipping takes 

 place at the attachments. The wire and rod are thus subjected to 

 twist, the amount of which is infinitesimal in the glass rod, which turns 

 round its axis practically as a rigid body j so that the whole twist may 

 be regarded as contained in the wire. This arrangement was adopted 

 to keep the vibrator, which in all cases was of iron (being selected 

 from a series of vibrators in the collection of apparatus), sufficiently 

 remote from the coil to obviate the possibility of any action between 

 the field of the coil and the moving vibrator, while the whole of 

 the twisting and untwisting material was contained within the field. 

 Round the lower edge of the vibrator is gummed a millimetre scale, 

 which is read by a telescope, and the vibrations were always sufficiently 

 slow to allow of the scale being read at the beginning and end of a 

 semivibration, the difference between the two readings giving the 

 amplitude. The coil consisted of 3080 turns of copper wire wound 

 on a double core of brass tubing. This double core was made of two 

 coaxial tubes, the outer 1J inches and the inner J inch in diameter. 

 The space between the tubes forms a water-jacket, through which a 

 stream of ^vater can be forced so as to shield the wire from the 

 heating effect of the current in the coil. The current produced by 

 the battery E, and adjusted by the resistance D, was measured by a 

 Kelvin graded galvanometer F, and the field was calculated from 

 the value 4;TrCn for the intensity of the field produced by a current 

 of C absolute units flowing in a coil of n turns per cm. of length. 



Results for Nickel. — The wire used was obtained from Messrs. 

 Johnson and Matthey, London, and was stated by them to contain 

 only a very small percentage of impurity. Its diameter was 1*4 mm. 

 The results of the experiments are shown in Diagram II. In the curves 

 of that diagram the abscissae, drawn from left to right, show amplitude 

 of vibration in degrees ; the ordinates, drawn downwards, represent the 

 number of periods which have elapsed from the beginning of the set 

 of observations. Thus we obtain the amplitude left after different 

 numbers of oscillations have been performed, and are able to estimate 

 from the curve the rate of subsidence. The ordinates may be taken 

 as giving the time from the commencement of the set of observations, 

 as it was found that the alteration of period produced by the imposi- 

 tion of the field was negligible, the actual periods being 7 '21 seconds 

 with a field of 132 C.G.S. and 7*17 seconds with no field. It will be 

 seen that the effect of small fields is to greatly increase the rate of sub- 

 sidence, but that at a field of about 160 C.G.S. the maximum effect of 

 the field in increasing the rate of subsidence is produced : thu3 



