i 9 °5.] McCLELLAN— USE OF OSCILLOGRAPH. 169 



falls on the three mirrors of the galvanometer, and is reflected to 

 the cylindrical lens CL. This renders the parallel ribbon of light 

 a point. The lens is focussed so that this point is in the plane of 

 the photographic plate. The chute through which the plane is 

 dropped is about ninety-five centimeters long, giving the plate a 

 speed which allows the record of one twenty-five cycle wave on a 

 four by five plate. Light enters the chute through a slit which is 

 provided with the hand shutter S. The shutter is open when the 

 plate is dropping, but is closed before the plate is pulled back to 

 the top so that the slide may be inserted. This prevents any possi- 

 bility of fogging. The plate is carried in an ordinary wooden four 

 by five plate holder. This in turn is held by a light wooden car- 

 riage H, which is provided with springs on the sides and back. 

 These are adjusted so that they just bear on the surfaces of the 

 chute, thus providing a very steady motion of the plate during the 

 fall. The springs also serve the purpose of holding the front of 

 the plate holder tight against the chute. The plate is started by 

 means of a bulb release R, and is stopped by an airdash pot D.P. 

 The bottom of the carriage is provided with a leather packed brass 

 piston which fits the cup. Light is provided by a powerful 25- 

 ampere arc light, which has the usual condensers. In addition, to 

 get a proper parallel beam, the concave lens L is provided. The 

 whole camera is provided with leveling screws, in addition to those 

 for the galvanometer. This is necessary, since the chute must be 

 vertical. The low potential currents used in this work were brought 

 to the galvanometer by lamp cords, which passed through corks in 

 the side of the box. For convenience in lifting the plate holder 

 and carriage the sliding bracket F is provided. This is raised by 

 the knob and string 7\ lifting the carriage until it catches in the 

 release apparatus. The bracket then drops to the bottom of the 

 box. It is held in place by a rod, on which it slides. For access 

 to the camera, the whole top of the box is arranged to slide in a 

 light tight groove. 



To adjust the apparatus for a curve the chute is first made per- 

 pendicular by the outside levelling screws. Then the galvan- 

 ometer is levelled with its own screws. The arc light is then 

 adjusted so that a strong beam of closely approximate parallel light 

 falls on the mirrors. The galvanometer is then adjusted, if need 

 be, so that the images pass through the center of the slit. Since 



