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. II.) 



Fig. 12. — Details of the ball-dropping mechanism. The steel 

 the tipping bucket D. which falls under the weight of the ball: 

 the register. 



passes through the valve A into 

 d closes an electrical circuit at E to 



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 



