264 HENRY A. EOWLAND 



in pairs first one direction of rotation and the other immediately after- 

 wards, everything except the rotation being kept constant. 



The table shows that, in every case except one, the deflexion for 

 negative rotation is appreciably smaller than the corresponding positive. 



The difference is too great to be due to accidental errors in the read- 

 ings, as the following table, giving the successive deflexions in the case 

 of #13 and #14 will show. 



There is but one deflexion in #13 as small as the mean of #14, and 

 but one in #14 as large as the mean of #13. 



This is a fair example of the way the deflexions run. As a further 

 illustration of this take#17 and#18; these two are identical in arrange- 

 ment, but the direction of rotation is in one case got by crossing the 

 belts from the countershaft to the disks and leaving the main bolt 

 straight; in the other the main belt is crossed while the auxiliary belts 

 are straight. The deflexions are the same. This, too, shows that the 

 difference cannot be due to any effect of the countershaft. The cause 

 of this has not yet been explained. The work is to be continued with 

 this and also with new apparatus, made like the Berlin apparatus, but 

 with the disk much larger, 30 cm. in diameter; at least double the 

 speed then obtained will be used. This ought to give deflexions on 

 reversal of 1-5 to 1-7 cm. 



The values of V do not agree so well as might be looked for; but. 

 when, in addition to the numerous difficulties already mentioned, the 

 smallness of the deflexion is considered, and the possibility of the needle 

 being affected by currents or magnets in other portions of the labora- 

 tory, so far away as not to be guarded against, and which might well be 

 changed between the time of taking the observation and the determin- 

 ation of the needle-constant, and, finally, that a distubing cause of some 

 kind is still undoubtedly present, the agreement is seen to be as good 

 as could justly be expected. 



Physical Laboratory, Johns Hopkins University, 

 April 22, 1889. 



NOTE, added April 29 



There seems to be a misunderstanding in certain quarters as to the 

 nature of the deflexion obtained in Prof. Eowland's first experiment. 

 The paper reads : " The swing of the needle on reversing the electri- 

 fication was about 10 to 15 mm., and therefore the point of equilibrium 

 was altered 5 to 7-5 mm/' This has been construed to mean that the 



