258 Prof. G. Wiedemann on the Methods 



lead by the employment of more powerful field-magnets. In 

 all cases the lead must be ascertained by its being the position 

 of least sparking, and the brushes set pretty closely to it ; 

 otherwise the brushes are burned away, and the sparking ia 

 unpleasant. 



Having charged the accumulators, I divided them up into 

 circuits as required, and employ them to work my electro- 

 pneumatic and other appliances. I have driven every thing 

 for a week after a charge of a couple of hours; but on account 

 of the local action in the red-lead cells, this is not a very advan- 

 tageous course; and latterly I have preferred to charge for 

 half an hour in the morning of each day. If the charge is 

 meant to last, of course the currents must be used economically. 



There are considerations, however, connected with the in- 

 evitable variation in the power of the current when any descrip- 

 tion of cells is used as a source of electricity, which make me 

 already doubtful whether this alternate procedure will answer, 

 and whether it may not be necessary to use the cells with the 

 machine running simply for the purpose of sluicing off small 

 currents with fairly constant differences of tension at their 

 sources from the main current of the machine. It is even pos- 

 sible that simple pairs of lead plates in acid without prepara- 

 tion may be sufficient for this purpose. But the discussion of 

 this matter must wait until my uniform rotation -machines are 

 in a more complete state. 



I will close with an experiment showing, in a manner suitable 

 for the lecture-table, the development of a counter electromo- 

 tive force in the dynamo when driven by a current from the 

 accumulators. 



I place a Swan lamp in parallel circuit with the machine. 

 Then, so long as the machine is at rest, the lamp does not burn. 

 But when the machine is in motion, as the velocity increases 

 the counter electromotive force is set up, and the passage of 

 the current through the machine more or less completely 

 barred. The lamp in the parallel circuit then burns up. If 

 the machine is stopped by a brake or overloading, the lamp is 

 extinguished again. 



XXX. On the Methods employed for determining the Ohm. 

 By G. Wiedemann*. 



THE Congress of Electricians which met at Paris in the 

 autumn of last year adopted the electromagnetic Units 

 based on the centimetre-gramme-second system for the con- 



* Translated from the Elektrotechnteche ZeiHchrift, July 1882. From 

 a separate impression, communicated by the Author. 



