﻿740 Prof. Hackett on Relativity -Contraction in a Rotating 



Conclusion, 



The effective capacity of a pancake coil Las been calcu- 

 lated, and the calculations have been verified experimentally 

 in two cases. 



The results of the calculation are that the capacity of 

 the coil 



when grounded at centre is 0*567 Ka /z//,f, 



when grounded at periphery is ... 0*330 Ka /x/xf, 

 when insulated is 0*252 Ka /z/zf, 



where a is the radius of the coil and K is the dielectric 

 constant of the medium. 



Washington, D.C.. 

 Jan. 14, 1922. 



LXIV. The Relativity- Contraction in a Rotating Shaft moving 

 with Uniform Speed along its Axis. By Felix E. 

 Hackett, M.A., Ph.D., Professor of Physics, College- 

 of Science for Ireland, Dublin *. 



§ 1. Introduction and Summary. 



A SOLUTION is offered in this paper of the problem of 

 . the relativity-contraction in a rotating shaft moving^ 

 with uniform velocity along its own axis. The standpoint 

 adopted is that of the fixed sether and the FitzGerald-Lorentz 

 contraction combined with the restricted principle of rela- 

 tivity. The validity of Euclidean geometry is assumed 

 throughout the paper. 



A hypothetical modification of Fizeau's method for 

 measuring the velocity of light is considered — a rotating 

 shaft carrying two disks with apertures which correspond 

 to the toothed wheel in Fizeau's experiment. It follows- 

 readily that when a rotating shaft is moving with uniform 

 velocity along its own axis, to a stationary observer, looking 

 in the direction of motion, it appears twisted in the opposite- 

 sense to the rotation. This effect has been pointed out by 

 R. W. Wood j, and he has discussed the experiment, but not. 

 in a sufficiently precise way to serve as a basis for the- 

 subsequent discussion in this paper. 



The arrangement may act as a clock. It measures time 



* Communicated by the Author. Read at the meeting of the British 

 Association, September ]921. 



t Wood, ' Physical Optics,' 2nd edit. p. 690. 



