320 How to excite more effettually [Book IV. 



To excite, therefore, an electrical machine effec- 

 tually, we muft firft find out thofe parts of the cufhion 

 which are prefTed by the glafs cylinder, then the amal- 

 gam muft be applied to thofe parts only. The line of 

 contact between the cylinder and cumion muft be 

 made as perfect as poflible, and the fire which is col- 

 lected muft be prevented from efcaping. The breadth 

 of the cumion mould not be great, and it mould be 

 placed in fuch a manner that it may be eafily raifed or 

 lowered. 



In order to find the line of contact between the cy- 

 linder and cumion, place a line of whiting, which has 

 been difiblved in fpirits of wine, on the cylinder ; on 

 turning this round, the whiting is depofited on the 

 cumion, and marks thofe parts of it which bear 

 or rub againft the cylinder. The amalgam is to be 

 put, on thofe parts only which are thus marked by the 

 whiting. 



Whenever the electricity of the cylinder grows Jefs 

 powerful, it is eafily renewed by turning back the fiik 

 which lies over it, and then rubbing the cylinder with 

 the amalgamated leather, or by altering the prefibre 

 of the adjufting fcrew. 



A fmall quantity of tallow placed over the amal- 

 gam is obferved to give more force to the electric 

 powers of the cylinder ; or the fame end may be 

 effected by rubbing the cylinder with a coarfe cloth, 

 which has been a little greafed, and afterwards wip- 

 ing the fame with a clean cloth. 



As air not only refifts the emiflion of the electric 

 fluid, but alfo diffipates what is collected, on account 

 of the conducting fubftances which are floating in it, 

 a piece of black or oiled filk mould be placed from 

 the line of contact to the collecting points of the prime 



conductor, 



