532 SCIENCE PROGRESS 



and similar branches of the subject are usually introduced. In 

 this manner electrostatics enters into the general study of elec- 

 tricity on the same footing as these other branches, according 

 to its intrinsic merits and loses the preferential position it has 

 held hitherto. 



The object of the course is to give some knowledge of electric 

 charges, potential on a more mechanical basis, the capacity of 

 condensers and induction. The source of electricity throughout 

 is the live wire of direct current supply mains (or, if they 

 are not available, of a battery of about seventy-five Leclanche 

 cells), the other main being usually earthed ; a resistance of the 

 order of 10,000 ohms should be put in the live supply main, so 

 that no harm is done by earthing it. We first demonstrate the 

 mechanical attraction between a charged body and " earth " by 

 means of two gold leaves hanging side by side as in an electro- 

 scope but insulated from one another and about 3 mm. apart j 

 one is connected to earth and the other to the main. This illus- 

 trates the principle of the ordinary commercial type of electro- 

 static voltmeter which is now introduced and used henceforth ; 

 the Ayrton-Mather type is found most suitable but the con- 

 struction of a Kelvin multicellular voltmeter is more open to 

 inspection. In either case the principle of its action can easily 

 be understood from a diagram or model. Charges are next 

 shown to be transferable by a portable conductor from the 

 source of supply to the terminal connected with the movable 

 plate of this instrument (the other terminal being earthed as 

 usual) ; the movement of the pointer shows an increase of 

 attraction with increase of charge. If it be desired it can be 

 shown that such a carrier will take no charge from the inner 

 surface of a hollow body connected with the live main ; and for 

 rough quantitative purposes the charge on a particular carrier, 

 after touching the main, may be treated as a unit charge. 



If the voltmeter be directly connected to the mains and the 

 electromotive force increased or diminished by additional cells, 

 the attraction is seen to vary with the electromotive force of 

 the charging battery. Before this stage the boy will have 

 become accustomed to the idea that the electromotive force of 

 the source of energy is distributed round the circuit and he uses 

 the phrase " potential difference between points on a current- 

 bearing conductor " to express this idea. We now take him a 

 step further towards a mechanical conception of electric poten- 



