300 E. B. Rosa — Determination of ' v, the Ratio of the 



the quantities which are required are the period of the vibrator 

 and the values of three resistances, quantities which are capa- 

 ble 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. Row- 

 land. It consists of a hollow sphere whose radius is 12*7 cm. 

 and within which may be hung either of two balls of 10 - 1 and 

 8*9 cm. radius, respectively. The condenser 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 mm. in diameter, in the outer shell, and attached to the insu- 

 lated end of a pivoted beam and counterpoised. By means of 

 a rack and pinion movement and vernier the ball may be accu- 

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

 form of a 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. 305), and Electrostatic 

 Capacity (p. 305). 



2. Galvanometer. — This was one of Elliott Bros.' Thomson, 

 high resistance, astatic galvanometers, made very sensitive. 



3. Timing forks. — Two of Konig's forks were used, whose 

 frequencies were approximately 32 and 130 per second. They 

 were driven by three or four Bunsen cells, the same current in 

 the case of the slower fork operating the vibrator P (fig. 1). 

 Their exact periods were determined by Michelson's method. f 



4 Vibrators. — The oscillating piece P in the case of the 

 slower fork was a commutator, such as that used by Thomson.;}: 

 The action of this form of a vibrator was regular and satisfac- 

 tory in the case of the slower fork ; but with the higher fork 

 great difficulty was experienced in obtaining sufficient uni- 

 formity, and finally it was abandoned and the following plan 

 devised as a substitute. T, T' (figs. 2 and 3), are two prongs 

 of the tuning fork, driven by the electromagnet M ; the inter- 

 rupter, attached to the end of one of the prongs, not being 



* Vol. i, p. 321. -f This Journal. Jan., 1883. 



% Thomson. Phil. Trans., 1883, or Glazebrook, Phil. Mag., vol. xviii, p. 98. 



