OP ELECTROLYTfiS — MCKltfRtCli. 38'7 



current be strong, for then the motions of the spot become 

 so rapid that, unless the double deflection is distinctly made, we 

 are not able to observe it. This difficulty I completely removed 

 by placing a rectangular sheet of zinc three or four inches 

 in front of, and hiding from my eye, the left side of the 

 screen. It was easy to place this so that, on looking past its 

 right edge, I could just see the faintest glimmer of the spot of 

 light. Since I always sent the current in such a direction that 

 the double deflection was to the right, it is evident that even the 

 least tremble of the spot to the right could be at once observed. 

 More especially was this so, as the sheet of zinc was much nearer 

 to the spot of light than to my eye. It is necessary, of course, 

 that tlie edge of the spot of light that we are observing should 

 be very clearly defined, and that no motion of the head should 

 be made at the instant of observation. 



(3.) With strong currents electrolysis of the solution goes on 

 at a rapid rate, and, again, there is danger of the coils becoming 

 heated. During the flnal measurements, then, when we are using 

 the strongest currents, contact in the battery wire must last only 

 for a small fraction of a second. The " rocker," thus, became too 

 slow a means of making and breaking contact. By joining up the 

 Galvanometer first and by a separate key (a method to be justified 

 later) the solution of this difficulty became easy, for then we have 

 only to make and break contact rapidly in the one wire. This I 

 accomplished by slightly flattening a piece of thick copper wire, 

 and sinking it into a board till its flattened surface was only 

 slightly above that of the board. This wire I connected with 

 one of the battery wires. The end of the other battery wire I 

 flattened, and by simply drawing it over the board and across 

 the wire, contact could be made and broken as rapidly as desired. 



I found it quite impossible, as did Ewing and MacGregor, and 

 Professor MacGregor, to obtain platinum electrodes which, in 

 themselves, did not constitute a voltaic cell. In every case my 

 electrolytic cell was found to be a voltaic cell with a small and, 

 apart from polarization, practically constant E. M. F. It is evi- 

 dent that, with this method, the adjustment of the arms which 

 would give no double deflection, would not be exactly the same 



