ELECTRICAL APPARATUS. 259 



Fig. 274. 



and tediousness of establishing a communication between 

 the insulated cover and the earth by means of the finger. 



To use the electrophorus, rub the upper surface of the 

 resinous plate with a piece of dry fur (cat's skin is reckoned 

 the best), and it will be excited negatively. Place the upper 

 conductor upon it, and then raise the same by its insulating 

 handle ; it will be found to exhibit very faint, if any elec- 

 trical signs. Replace the conductor, and whilst it lies on 

 the surface of the excited plate, touch it with a finger or 

 any other uninsulated conductor, and then raise it again by 

 its handle. 



It will now be positively electrified, and afford a spark ; 

 if it be then replaced on the resinous plate, touched, and 

 again raised, another spark will be procured, and this pro- 

 cess may be repeated for a considerable time without any 

 perceptible diminution of effect. Jars may be charged by 

 bringing them in contact with the conductor each time it is 

 lifted ; with an instrument of this kind, only six inches in 

 diameter, Cavallo charged a jar several times successively, 

 and such was the strength of the charge that it was capable 

 of piercing a card. 



This instrument, properly constructed, has been known 

 to retain its electricity so long as three weeks, without requir- 

 ing fresh excitation. 



Price, $3.00 to $6.00. 



