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Mr. C. W. Siemens on certain means of [Jan. 30, 



is only necessary, therefore, to be able to count the number of gradua- 

 tions which pass in that period. As a perfectly uniform rotation has 

 not been obtained, a regulator under the control of the operator is 

 employed. This consists merely of a piece of string which passes 

 round the axis of the drum, and also round a pulley which can be 

 turned by the operator's left hand. An upward or downward motion 

 of the wave denotes that the drum is going too fast or too slow, and 

 by means of the pulley a gentle check or acceleration sufficient to keep 

 the wave steady is given to the drum. 



An electric counter gives the number of complete revolutions accom- 

 plished by the drum in any given period, and a fine-pointed tube, 

 containing magenta, is carried by a saddle above the drum, and being 

 actuated by an electro-magnet, makes a dot on a piece of white paper 

 wrapped round the drum at the beginning and end of the experiment. 

 The distance apart of these dots gives the additional fraction of a revo- 

 lution accomplished by the drum during the period of the experiment. 

 Electric circuits are so arranged that a reverser turned a few seconds 

 before the minute at which it is intended to begin the experiment, 

 causes a current to be sent exactly at that minute by the clock relay, 

 which starts the electric counter, and also makes a dot on the drum. 

 Just before the expiration of the last minute of the experiment, the 

 reverser is turned in the opposite direction, and at the expiration of 

 that minute the counter is stopped, and a second mark made on the 

 drum. 



Some of the results obtained with different forks are given. 



The results of further experiments made to determine the effect of 

 temperature, of continuous and intermittent bowing, and of the mode 

 of fixing the fork are appended. 



An optical method by which two slightly dissonant forks may be 

 compared without altering the period of either, is described. 



Figures and diagrams fully explaining the apparatus employed, 

 accompany the paper. 



IV. " On certain means of Measuring and Regulating Electric 

 Currents." By C. AVilliam Siemens, D.C.L., F.R.S. Re- 

 ceived January 16, 1879. 



[Plates 4, 5.] 



The dynamo -electric machine furnishes us with a means of pro- 

 ducing electric currents of great magnitude, and it has become a 

 matter of importance to measure and regulate the proportionate 

 amount of current that shall be permitted to flow through any branch 

 circuit, especially in such applications as the distribution of light and 

 mechanical force. 



