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Igi2] BOVIE—PRECISION AUXANOMETER 505 
The mechanism may be understood by referring to fig. 1. The 
plant is attached by the invar wire a to the small spring 6, which 
pulls upward a little more than is necessary to lift the weight of the 
wire (the exact amount of this pull may be regulated easily by a 
screw not shown in the drawing). Experiment has shown that this 
slight pull is not enough to affect the growth of the plant. 
As the spring moves upward, it comes in contact with the block 
c at the point c’. This closes the electric circuit, which up to this 
time has been open, since the spring 0 is insulated from c at its other 
point of contact. The current which now flows through the line 
energizes the coil d, which draws the escapement lever upward. 
The block ¢ is carried on the screw f, which is connected by a train 
‘ of wheels at f’ to a clock spring, which tends to turn the screw in 
such a manner as to raise the block c upward. The screw is pre- 
vented from turning by the escapement lever e, but when the 
-escapement lever is drawn up by the magnet d, it allows the screw 
to turn a given part of a revolution. The block c is therefore 
raised a certain distance and the circuit opened. The plant must 
now grow exactly this distance before the circuit is closed again. 
The upward movement of the block c is determined by the pitch 
of the thread of the screw f and the amount it turns. By changing 
the number of teeth in the escapement wheel at the top of the screw 
jf, the amount it turns at each contact can be controlled. There are 
20 teeth in the escapement wheel, and so the screw can be made to 
turn 1/20 of a revolution, or if part of the teeth are removed, it 
will turn more, up to a complete revolution. If the pitch of the 
screw is o. 5 mm., each record represents 25. 
It may be noted, in passing, that the micrometer screw has 
become the standard device for measuring small distances. It is not 
the purpose of this account to give the mechanical specifications of 
the apparatus; there are many ways in which the various details 
can be constructed. It may be said, however, that the screw should 
be straight and accurate. (The screw from a phonograph, with its 
nut, is an excellent and inexpensive micrometer.) It should turn 
easily and run true in its bearings, and the nut in the block c should 
be so constructed as not to bind. In this machine use is made of 
a split nut which is tightened by a conical cap into which it fits. 
