Ratio of the Electromagnetic to the Electrostatic Unit. 317 



the J capacities, and any error in the latter enters into v by 

 only half its amount. 



There are several important advantages of this method of 

 measuring the electromagnetic capacity. In the first place a 

 knowledge of the exact electromotive force and resistance of 

 the battery is not required, and their constancy is not essential. 

 In the second place, since it is a null method, such uncertain 

 quantities as logarithmic decrement, torsion of the suspending 

 fibre, and period of the needle are not required; the galva- 

 nometer can readily be made more sensitive than a ballistic 

 galvanometer; its "constant" need not be known; and the 

 field of force may be variable both in intensity and direction 

 without prejudice to the experiment. On the other hand, 

 the quantities which are required are the period of the 

 vibrator and the values of three resistances, quantities which 

 are capable of determination to a very high degree of accuracy. 

 In the present case the vibrator was either a tuning-fork or 

 else it was driven by a tuning-fork, and by the arrangement 

 adopted the uncertainty in its period was reduced to an 

 extremely small quantity. The difficulties and limits of the 

 method will appear under the head of Sources of Error. 



Instruments. 



1. Condenser. — This was made from designs by Prof. 

 Rowland. It consists of a hollow sphere whose radius is 

 12*7 centim., and within which may be hung either of two 

 balls of 10' 1 and 8*9 centim. radius respectively. The con- 

 denser has a capacity of about 50 absolute electrostatic units 

 with the larger ball and 30 with the smaller. 



The spherical surfaces are accurately ground, nickel-plated 

 and polished to a mirror surface. The ball is suspended by a 

 silk cord C (fig. 3) passing through a hole 7 millim. in 

 diameter in the outer shell, and attached to the insulated end 

 of a pivoted beam and counterpoised. By means of a rack 

 and pinion movement and vernier, the ball may be accurately 

 set in any desired position. Maxwell * objects to this form 

 of condenser on account of the difficulty of working the 

 surfaces accurately spherical, making them truly concentric, 

 and determining with sufficient accuracy their dimensions. 

 That these difficulties have in the present case been entirely 

 surmounted will, I think, appear from the discussion under 

 the heads of Displacement of the Ball (p. 323) and Electro- 

 static Capacity (p. 323). 



* Vol. i. p. 321. 

 Phil, Mag. S. 5. Vol. 28. No. 173. Oct. 1889. 2 A 



