126 
Journal of Agricultural Research 
Vol. V, No. 3 
an electromagnet, M (fig. 15). When the beam circuit is closed, the 
motor is released and raises the beam through a cam, K (fig. 14). When 
the cam shaft S (fig. 15) has completed one revolution, the arm H 
on the cam shaft again engages the spring R on the armature T of the 
magnet, and the motor is stopped. 
Adjustable poise for raising center of gravity of beam. —It is 
essential that the mercury contact on the beam be closed with a positive 
motion to avoid the fluttering of the relay armature. This is accom¬ 
plished by raising the center of gravity until the beam is slightly unstable, 
by means of an adjustable bob, W, located above the central knife-edge. 
(See fig. 11.) 
Fig. ia.—Details of the ball-dropping mechanism. The steel ball passes through the valve A into 
the tipping bucket D, which falls under the weight of the ball and closes an electrical circuit at E to 
the register. 
MarviN recorder. —A convenient type of recorder for registering the 
time at which each ball is delivered is that devised by Marvin for use 
in connection with automatic rain gages. This recorder has a drum, 12 
inches in circumference, which makes one revolution in six hours and 
is continously offset by a screw, so that the four 6-hour periods are 
recorded side by side on the same sheet. A valuable feature is a zigzag 
attachment on the magnet, by means of which the tracing pen is perma¬ 
nently displaced each time the magnet circuit is closed. This gives a 
record which is much easier to read than the ordinary record in which 
the pen returns to its initial position when the circuit is opened (fig. 16). 
The dropping of two balls in rapid succession is easily seen in the zigzag 
