Intelligence and Miscellaneous Articles. 215 



work the rocker quickly to the left, and allow it instantly to fall 

 back again — a process w r hich need not occupy more than a small 

 fraction of a second, yet which must not be performed too quickly, 

 on account of the inertia (small as it is) of the needle and mirror of 

 the electrometer. If the deflection of the electrometer be suddenly 

 increased or diminished by this action, slide one of the wires B 2 

 along the wet string, a little further from or nearer to the other, 

 and rock again, — continuing this process till a charge is found which 

 leaves the electrometer at rest when the rocking to and fro is per- 

 formed. Reverse a commutator attached to the wires E, and repeat 

 the operation. The difference of the scale-readings in these two 

 cases gives a number proportional to the electromotive force of the 

 polarized plates — (I say difference, because the scales commonly 

 used with Sir W. Thomson's instruments are, to avoid confusion, 

 graduated from one end to the other, as they ought to be, instead 

 of being graduated opposite ways from the middle). To enable this 

 measure to be reduced to absolute units, a normal Daniell's cell was 

 applied at intervals, during each day's work, directly to the elec- 

 trodes of the electrometer, then reversed ; and the difference of the 

 readings was tabulated as representing its electromotive force. 



In the other experiments I used a plate of gutta percha in which 

 the ten holes were bored, but for a time discontinued its use on sus- 

 pecting that it sometimes led to irregular working of the apparatus 

 by imperfect insulation. The cups were then separately mounted 

 on insulators 3 inches high; but this was not found to be an im- 

 provement of any consequence, and the holes are now made in a 

 small, but thick, plate of vulcanite. 



In this note the numbers presented must be looked upon only as 

 first approximations ; but the apparatus has now been carefully con- 

 structed by an instrument-maker, and Mr. Dewar has begun an 

 elaborate series of experiments with it, from which valuable results 

 may soon be expected. In the trials which have as yet been made 

 we employed a temporary apparatus, rudely built up of wires, seal- 

 ing-wax, and gutta percha. We have rather been endeavouring to 

 determine whether the process, complicated as it is by the inertia of 

 the moveable part of the electrometer, the quickness with which the 

 rocking can be conducted, and the rate at which the polarization 

 begins to diminish as soon as the polarized plates are detached from 

 the decomposing battery, is capable of being made to give good re- 

 sults, than in actually attempting to get such. So far as I can yet 

 see, the first of these complications is alone likely to cause any 

 serious embarrassment ; and should such be the case, which I do not 

 anticipate, a form of experiment a little more laborious than that 

 above described, and which I have already once or twice tried, seems 

 to be well adapted to meet it. 



The following are, for the most part, means of a great number of 

 determinations. The electrolyte was usually dilute commercial sul- 

 phuric acid, 1 part acid to 10 of water ; and to the lead and other 

 impurities it was found to contain we may ascribe the fact that the 

 results were not very accordant from day to day, so that it was not 



