322 PROCEEDINGS OF THE AMERICAN ACADEMY. 



The function of Kj is to discharge the condenser through the galva- 

 nometer after K 2 has hroken circuit. The instrument would ordinarily 

 experience a constant deflection, but K a and K 2 are rigidly connected 

 and mounted on a radial arm, which is geared to the shaft so that it 

 moves very slowly. The effect is to gradually move the contact point 

 over the wave. The deflection of the galvanometer will at any instant 

 be proportional to the P.D. between a and b at the instant of breaking 

 at K 2 , or in other words, the deflection follows the wave form. 



The actual arrangement is shown in Figure 2 (see Plate), where the 

 contact device, the synchronous motor, and the direct current motor used 

 for starting the apparatus will be seen. By use of worm gearing the 

 wheel train necessary for moving the brushes is made very compact ; the 

 reduction for the instrument shown is 7200 to 1. 



I have found Sullivan's Universal Galvanometer to be a most satisfac- 

 tory instrument for use with the apparatus. This galvanometer, of the 

 D'Arsonval type, has a carefully balanced coil, so that it is not very 

 susceptible to mechanical disturbances ; also the magnetic damping is 

 most carefully adjusted. The instrument is not of great sensitiveness, 

 but owing to the stiff suspension the zero is perfectly definite. 



The camera used for recording the curves is shown in Figure 3 (see 

 Plate). The plate is contained in an ordinary plate holder. This is 

 moved vertically by a fine wire which is wound on a drum, seen in Fig- 

 ure 2, just in front of the lower worm-wheel. This drum can be thrown 

 in at pleasure by a pin clutch. The slide of the plate holder is held sta- 

 tionary by a pin, so that the plate is exposed as the holder is drawn up. 



The front of the camera, shown removed, is provided with a narrow 

 slit about , ,', ,, of an inch wide. In front of it are projecting lips 9 inches 

 long and | of an inch apart. They are blackened within and serve effect- 

 ually to shut out extraneous light, and thus prevent fogging of the plate 

 The spot of light used was the sharply focussed image of the filament of 

 an incandescent lamp. An alternative arrangement is to use a plate of 

 ground glass in the holder, and to have a straight-edge fastened across 

 the guides. It is then easy to keep the point of a pencil in contact with 

 it and upon tin- spot of light. 



Tin- arrangement described is of course a device for obtaining the 

 average wave, and unsuitable for recording transient phenomena. Tin' 



time taken in recording a wave at l"-'" cycles per second is about 1 ' 

 minutes. 



Tin- adjustable condenser allows one to adapt the apparatus to vary- 

 ing conditions, so the B.M.F. curves may be taken directly, ami the 



