OP ARTS AND SCIENCES. 123 



An inspection of these curves shows immediately that the greater 

 the strength of the field required to saturate the diaphragm, the greater 

 is the strength of the field at which the maximum current occurs. 

 Thus in Figure 1 the maximum current with Curve 1 corresponds to 

 a strength of field of about 12 units of the scale used, while with Curves 

 2 and 3 the corresponding strengths of field are 20 and 43 units re- 

 spectively. Also in Figure 2 the maximum currents will be seen to 

 correspond to greater strengths of field in proportion to the thickness 

 of the diaphragm, and in Figure 3 similar though less marked results 

 hold for tempered as compared with uutempered steel of the same 

 thickness. 



It would also be expected that the value of the maximum current 

 would be greater with a thick than with a thin diaphragm. This was 

 usually the case in our experiments. Thus the Curves 1, 2, 3, of Fig- 

 ure 1 give maximum currents of 27.5, 50.2, and G2.6 units respectively. 

 Results of a similar nature are shown by Figure 3, the maximum cur- 

 rent with the untempered diaphragm being far greater than with the 

 tempered one. Curve No. 23 of Figure 2 is apparently an exception. 

 It is probable, however, that this is in appearance only, and that the 

 three curves of that figure are not strictly comparable with one another. 

 The rigidity of the diaphragms here used, especially of the thicker 

 ones, is considerable, and any sliglit yielding of the supports of the rod 

 which carried the cam would prevent the actual throw of the diaphragm 

 from being as great when this had considerable thickness, and would 

 greatly diminish the strength of the current produced. 



The peculiarity of Curve 2, as compared with 1 and 3, is probably 

 caused by the want of both magnetic and mechanical continuity in the 

 material of the multiple plate formed by the several diaphragms used. 



In those cases where steel diaphragms were employed, there was' 

 always a notable induced current, even when the reading of the mag- 

 netometer was zero. This was probably due to a slight residual mag- 

 netization of the diaphragms. 



The results stated in this paper may serve to exjilain a phenomenon 

 which has seemed somewhat obscure. Frequent attempts have been 

 made to increase the efficiency of a magneto transmitter by polarizing 

 the diaphragm as well as the magnet, a common way of doing this 

 being to employ a horseshoe magnet one leg of which is in contact 

 with the edge of the diaphragm, while the other, about which the coil 

 is wound, is placed in its usual position opposite the centre. But as a 

 general rule little or no gain has seemed to result therefrom, so far as 

 can be judged by the performance of such instruments in actual prac- 



