318 Mr. E. B. Rosa on the Determination o/v, the 



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

 high-resistance, astatic galvanometers, made very sensitive. 



3. Tuning-Forks. — Two of Koenig'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 Michel- 

 son's method*. 



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

 slower fork was a commutator such as that used by Thomson f . 

 The action of this form of vibrator was regular and satis- 

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

 fork great difficulty was experienced in obtaining sufficient 

 uniformity, and finally it was abandoned and the following 

 plan devised as a substitute. T, T (figs. 2 and 3) are two 



Fig. 2. 



~-A 



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

 the interrupter, attached to the end of one of the prongs, not 

 being shown, b, V (fig. 3) are two fine brass wires, uniting 

 at n aria* tipped with platinum at p, p', where they are bent 

 at right angles and fastened to the fork with an insulating 

 cement. V, V are two small blocks of vulcanite attached to a 

 firm support A. Below the platinum points are two cavities 

 in the vulcanite which are filled with mercury, and as the 

 fork vibrates first one and then the other of the points dips 

 into the mercury. Thus the mercury cups, which are joined 

 to B and D respectively (fig. 1 ) answer to the posts S and R, 

 while the wires b, V unite and, passing through the fine 

 glass tube G ; reach the ball of the condenser at m. When 

 the prongs separate, p' dips into the lower cup and the 



* Phil. Mag. [5] xv. p. 84 (1883). 



t Thomson, Phil. Trans. 1883, or Glazebrook, Phil. Mag. [5] xviii. 

 p. 98. 



