580 THE ELECTROPHORUS. 



several weeks, when the cover is kept upon it ; nevertheless, if an 

 energetic action is desired, it should be electrified again each time 

 it is used. 



The cake should always be kept upon the form so that no portion 

 of the edge may project, for such projecting portions are apt to 

 become bent downward during the heat of the summer, although 

 the cake does not appear sensibly soft. In the summer the cover 

 should not be kept upon the cake, or it may possibly sink by its own 

 weight some little distance into it. 



The spark obtained from the cover while it rests upon the cake 

 produces a very sensible effect, when one finger is placed upon the 

 form and another finger, either of the same or of the other hand, is 

 brought near the cover. To obtain smart sparks from the raised 

 cover the experimenter must take care lest his clothes should come 

 too near the cover, for projecting points such as form the rough 

 surface of clothing diminish the quantity of electricity in electrical 

 bodies, as will be seen later on. For the same reason all other 

 bodies should be removed as far as possible from the electrophorus. 



When a rough body, for example, the knuckle of the finger, a 

 file, or a small piece of wood, is brought near, especially if the ap- 

 proach is slow, several small sparks are obtained from the raised 

 cover ; these sparks are scarcely visible and hardly audible, but 

 very bright and audible sparks are obtained if the body employed 

 is a metallic conductor well rounded at the end brought near; 

 the round handle of a pair of scissors, or still better, a brass ball 

 soldered to a stout wire, such as forms part of various electrical 

 apparatus, are very serviceable for this experiment. 



The sparks obtained from the edge of the cover are somewhat 

 longer than those obtained from the more central portion, but they 

 are less audible and bright, An electrophorus of the given dimen- 

 sions should in dry weather give sparks 2 cm long. 



As carrier or proof-plane a common toy-marble of glass or stone, 

 1 or l cm> 5 in diameter, may be used ; it is attached to an insulating 

 handle and then provided with a well-conducting surface by cover- 

 ing it first with a layer of gum and then with goldleaf. The insu- 

 lating handle is made of a small stick of shellac or sealing-wax, 

 4 to 8 mm thick and 8 or 10 cm long, formed by rolling soft shellac or 

 sealing-wax between the fingers ; it may also be made from the 

 substance of which the electrophorus-cake consists, by scraping with 

 the splinter of wood a sufficient quantity from the sides of the pot. 

 The knob is heated until the end of the little stick melts upon it ; 

 when cool it will remain fixed to it. 



A metallic disc fixed to the insulating handle will also serve as 



