342 Major Garde w. On a Differential Electrostatic [Jan. 21, 



commonly called the two kinds of electricity in any time, however 

 extended, and at any instant at an eqnal rate. 



I have recently drawn attention to this law as determining the 

 potentials from earth of the two sides of any system of electric supply 

 (vide my paper read on the 23rd April, 1891, at the Institution of 

 Electrical Engineers). 



If we conceive, therefore, a perfectly insulated voltaic battery, the 

 potentials of the terminals of this source from earth would be deter- 

 mined by momentarily connecting any one of the metal plates with 

 the earth. Under such conditions, the smallest leakage from either 

 pole to earth through a resistance amounting to many millions of 

 megohms, if unbalanced by any leakage from the other pole, must 

 rapidly reduce the potential of the imperfectly insulated pole to- 

 zero. 



The only limit, therefore, to the sensibility of the method is the 

 imperfection of the insulation of the measuring apparatus, and this 

 insulation, with proper precautions, can be easily maintained at a value 

 exceeding 1,000,000 megohms. 



Sources of E.M.F. A very convenient source of E.M.F. for the 

 purpose is the arrangement of small zinc-copper couples in series, 

 moistened by dipping the whole into a pan containing acidulated 

 water, which is in use at the Physical Laboratory at Glasgow 

 University. 



Four hundred such couples are usually arranged on an ebonite 

 support, and the sensibility with this number is ample.* 



A still better source, when alternating currents are available, is a 

 special form of transformer, the secondary coil being suspended in 

 air by a silk cord. 



The highest insulation can thus be secured. But when the resist- 

 ance to be balanced possesses appreciable capacity, the use of an 

 alternating E.M.F. is unsuitable, on account of the masking of the 

 effect of the leakage current proper by the capacity current. 



Comparison Resistances. The variable resistance is, most conveni- 

 ently, some material of uniform cross section, so that its resistance 

 varies as the length put in circuit. Reels of white silk, cotton, 

 thread, and string are very suitable, and with a few such simple 

 materials, balances can be obtained through a great ran*ge of value, 

 'although no multiplying or dividing power is possible. Thus, a 

 white embroidery silk has been found to have a resistance of approxi- 

 mately 250,000 megohms per inch ; a green thread, partly silk and 

 partly cotton, 10,000 megohms per inch in a dry atmosphere ; 



* The mahogany legs supplied with this battery should be replaced by grooved 

 ebonite legs, to improve insulation, and it is also of advantage to insert under each 

 leg a piece of sealing-wax. The couples require to be taken out and cleaned occa- 

 sionally; if allowed to get dirty, the E.M.F. becomes low. 



