130 Lord Kayleigh and Dr. A. Schuster. [May- 5, 



with the auxiliary magnetometer. A recomparison of resistance with 

 the standard completed the set. The magnet in the centre of the coil 

 should, when no current is passing through the coil, always go through, 

 the same changes as the magnet of the auxiliary magnetometer. If 

 this could be insured, the two might be compared once for all, or the 

 comparison might even be omitted altogether, for the difference between 

 the deflections of positive and negative rotations, when corrected for 

 changes in the earth's magnetism, would give the double deflection 

 independently of the actual zero position. Unfortunately, however, 

 and this was our greatest trouble, the comparison between the magnet 

 and the auxiliary magnetometer showed that we had to deal with a 

 disturbing cause, which rendered a frequent comparison between the 

 two instruments necessary. This disturbing cause, which we traced 

 to air currents circulating in the box containing the magnet,, will be 

 discussed presently. 



The observations were taken on three different evenings and one 

 afternoon. The evenings (8 h. p.m. to 11 h. p.m.) were chosen on 

 account of the absence of disturbances, which, daring the usual 

 working hours, are almost unavoidable in a laboratory. We may 

 give, as an example for the regularity with which the magnet vibrated 

 round its position of rest, a set of readings which were taken while 

 the coil revolved about four times in one second, the circuit being 

 closed. 



T=9 h 36 m . £=13°'0 C. 



Rotation. 



Negative. 



374 -4 



362 -1 



373 -3 



362 -8 



372 -2 



362 -0 



373 -9 



361 -4 



372 -8 



362 -0 



372 '8 



362 -0 



372 -4 



363 -8 



371-8 



364 -0 



371-1 



364 -0 



370-5 





372 -52 



362 -68 



Position of rest, 367 "60. 

 T = 9 h 38 m -5. *=13°-0 C. 



The number of readings taken were not always the same, but 

 varied generally between sixteen and twenty. 



We used, in the course of our experiments, four different speeds. 

 The method of obtaining and regulating these has been explained- by 



