2IO REPORTS OF INVESTTCATIONS AND PROJECTS. 



(2) Experiment proper: Initially, i and 5 are open, 2, 3, 4 closed, H 

 down, the rotor at rest, / zero. Then, in succession, H is raised, the motor 

 driving the rotor is started, the coil circuit is closed for 30 seconds, the 

 ammeter is read, 3 is opened, and immediately the coil circuit is broken, the 

 motor is stopped, and the electrometer scale-reading is taken. The process 

 is repeated with the current in the coil reversed. The double deflection d is 

 thus obtained. Speeds are recorded automatically on a chronograph. 



For the experiment as now performed, Lorentz's theory, as will be shown 

 in the complete paper, gives the following relation: 



'/»'(^> + *' + ^)7 W 



K being expressed in the B.S. unit, T in the second, and the remaining elec- 

 trical quantities in the practical units. This equation may also be written 



^K.-(.' + K)=^* (.) 



In the case of sulphur, the mean value of the first member of (2) differed 

 from the second member by but a small fraction of i per cent, although the 

 separate values obtained for the first term differed much more from their 

 mean. In the case of rosin, V has not yet been determined, a different rotor 

 having been used. The rotors are nearly alike, however, and it is certain 

 that (2) is approximately satisfied by the experiments with rosin. For both 



sulphur and rosin, — V^ has been found proportional to ^as required by (i ). 



This, however, would be expected on any theory. The agreement of the 

 experiments with equation (i) or (2) forms a striking confirmation of 

 IvOrentz's theory, according to which the aether remains at rest when matter 

 moves in it, and the matter, electrically constituted, is therefore alone acted 

 upon by the motional electric intensity. 



In an investigation on this subject begun much later than this one, but 

 completed in 1904, Dr. H. A. Wilson,* of Trinity College, Cambridge, with 

 the assistance of Professor Larmor, produced a formula for his experiment 

 which is incomplete and incorrect. The correct formula is, suitable units 

 being used, 



^ ^{K-i)S,<l> + S{<i>' + V+ V) + S' V']^L (3) 



Here <^, </>', V, I, T, and K have the same meanings as in equation (i) ; 

 5*0 is the capacity the ebonite cylinder used by Wilson would have if the 

 ebonite were replaced by sether, its electric field confined to the region 

 between the coats, and the lines of intensity all straight and radial ; 5* is the 



d 

 9o 



* H. A. Wilson, Roy. Soc. Phil. Trans., Series A, 1904. 



