CHAP, i.f ELECTRICITY AND MAGNETISM. 41 



devised. All electrical machines consist of two parts, 

 one for producing, the other for collecting, the electricity. 

 Experience has shown that the quantities of + and 

 electricity developed by friction upon the two surfaces 

 rubbed against one another depends on the amount of 

 friction, upon the extent of the surfaces rubbed, and also 

 upon the nature of the substances used. If the two 

 substances employed are near together on the list of 

 electrics given in Art. 5, the electrical effect of rubbing 

 them together will not be so great as if two substances 

 widely separated in the series are chosen. To obtain 

 the highest effect, the most positive and the most 

 negative of the substances convenient for the construc- 

 tion of a machine should be taken, and the greatest 

 available surface of them should be subjected to friction, 

 the moving parts having a sufficient pressure against one 

 another compatible with the required velocity. 



The earliest form of electrical machine was devised 

 by Otto von Guericke of Magdeburg, and consisted of 

 a globe of sulphur fixed upon a spindle, and pressed 

 with the dry surface of the hands while being made to 

 rotate; with this he discovered the existence of electric 

 sparks and the repulsion of similarly electrified bodies. 

 Sir Isaac Newton replaced Von Guericke's globe of 

 sulphur by a globe of glass. A little later the form of 

 the machine was improved by various German electri- 

 cians ; Von Bose added a collector or " prime con- 

 ductor," in the shape of an iron tube, supported by a 

 person standing on cakes of resin to insulate him, or 

 suspended by silken strings ; Winckler of Leipzig sub- 

 stituted a leathern cushion for the hand as a rubber ; 

 and Gordon of Erfurth rendered the machine more easy 

 of construction by using a glass cylinder instead of a 

 glass globe. The electricity was led from the excited 

 cylinder or globe to the prime conductor by a metallic 

 chain which hung over against the globe. A pointed 

 collector was not employed until after Franklin's famous 



