1866.] 



Indications of Animal Electricity, 



157 



detect these latter phenomena easily and satisfactorily until I hit upon the 

 method of investigation employed in the experiments about to be de- 

 scribed — a method which dispenses with the use of the condenser, which 

 appears to be as delicate as it is certain and simple, and which I now pro- 

 ceed to describe without any further preamble. 



I. An account of the method of experimenting employed in the expe- 

 riments which have to be related presently . 



In order to carry out this method of experimenting, the instruments 

 necessary are two small electroscopes, two insulating stands upon which 

 to fix these electroscopes, and a conducting rod with an insulating handle. 

 Each electroscope is provided in the usual way with a pair of gold leaves, 

 and with slips of tinfoil in the interior of the glass bell of the instrument, 

 and it has, in addition, an opening underneath the wooden base, by which 

 it may be screwed on the top of the insulating stand. Each insulating 

 stand is a piece of glass rod 9 or 10 inches in length, fixed by its lower 

 end into a suitable foot, and having at the upper end a screw which fits 

 into the opening underneath the wooden base of the electroscope. In one 

 stand (for reasons which will appear presently) the glass stepn is varnished ; 

 in the other it is left unvarnished. The conducting rod may be of any 

 form. For the rest, all that need now be said is that, in order to avoid the 

 chance of electricity being developed by the friction of lackered surfaces, 

 the caps of the electroscopes, and the end of the conducting rod which 

 has to be brought into contact with the caps at certain times, are left 

 unlackered, and that, in order to secure as good insulation as possible, the 

 exterior of the electroscopes are well varnished whenever practicable. 



In preparing for an experiment, the electroscopes are screwed on the 

 insulating stands, and then charged in a particular way with free electricity — • 

 the one with free positive electricity, the other with free negative electricity. 

 This charge is obtained by gently rubbing the glass stem of the insulating 

 stands between the finger and thumb — the positive electricity from the 

 unvarnished glass stem, the negative from that which is varnished. The 

 electricity thus obtained is communicated, not to the cap, which is in 

 direct communication with the gold leaves, but through the wooden base 

 to the tinfoil slips which run halfway up the interior of the glass bell of 

 the electroscope ; and thus, instead of being charged directly, the gold 

 leaves become charged inductively with the opposite kind of electricity to 

 that which is communicated to the tinfoil slips. The result of doing this 

 is that the gold leaves take up a given degree of divergence, and that they 

 remain divergent so long as the tinfoil slips retain their charge of electricity. 

 Charged in this m.anner, in fact, the gold leaves cannot be brought 

 together by placing a conductor between the cap of the electroscope and 

 the earth ; indeed, so far from this being possible, the effect of placing a 

 conductor in this position, under these circumstances, is (as may easily be 

 understood) to increase the divergence of the gold leaves. 



