578 THE ELECTROPHORUS. 



in its turn the negative electricity of the surface of the 

 cake, and thus prevents the dissipation of the charge 

 which takes place while the cover is removed, in conse- 

 quence of the access of air, which acts as a conductor. 

 As long as the cover is on, it obviously excludes the air, 

 and the mutual action of cover and cake is preserved. 



The form and the cover should be made by a tinsmith, the form 

 of tin and the cover of sheet zinc or brass. The form is a flat round 

 mould 20 cm in diameter and l cm< 5 deep ; the cover is a round flat 

 disc, 16 cm in diameter, with a solid rim which is formed by bending 

 the edge all round over a stout wire, as shown in section at _B, fig. 

 310. Much care must be taken to make the disc perfectly flat and 

 the rim nicely round and smooth. Small loops of the form shown 

 at C in the figure, made of brass- wire l mm thick, are soldered 

 at three points equally distant from each other, and 2 cm from the 

 edge. To each loop one end of a thin cord of silk is tied, each cord 

 being 15 cm long, and the other ends are joined by a common knot. 

 The cords must be of pure silk, perfectly free from cotton, or they 

 will not insulate properly. If pure silken cords cannot be obtained, 

 three or four threads of common sewing silk may be used instead of 

 each cord. Or loops and cords may be altogether dispensed with by 

 attaching an insulating handle to the disc ; it is first heated and the 

 end of a stick of sealing-wax pressed upon it. The sealing-wax 

 handle is more convenient than the cords but is rather easily 

 broken. 



The melted resinous matter for the cake must not be poured into 

 the tin mould. The latter is placed bottom upwards upon a steady 

 even table and surrounded by a rim of paper. A few strips of stiff" 

 writing paper, 3 cm wide, are pasted together at the ends so as to 

 form a strip 70 or 80 cm long, which will pass more than twice round 

 the form. It is laid tightly around the rim, so as to project l cm '5 

 above the bottom of the form and the end is fixed with gum. Forty 

 grammes of yellow bees' wax and 40 grammes of turpentine are next 

 heated in an earthenware pot of about 1 litre capacity. Turpentine, 

 which must not be confounded with oil of turpentine, is a thin resin- 

 ous liquid, of which two sorts are usually sold, the common and the 

 Venice turpentine ; the former, which is inferior, will do for our 

 purpose. The heating should be very slow and gradual, or the pot 

 will crack, and the mass should be constantly stirred with a splinter 

 of wood. When all the wax is melted, 400 grammes of shellac are 

 added in small portions, each time about a handful of the thin scales 



