A PRECISION AUXANOMETER 
Wz. T. BoviE 
(WITH TWO FIGURES) 
In the auxanometers which have been described up to the present 
time, a thread or string has been used to transmit the motion of the 
growing plant to the recording device. This construction is always 
faulty, because changes in humidity affect the length of the thread, 
and so falsify the record. The author has designed a machine 
which eliminates the thread and its unavoidable errors by sub- 
stituting for it material which is not affected by humidity and 
which is very little altered by changes in temperature. 
On account of the error due to the thread, it has heretofore been 
impracticable to use a recording device which would indicate small 
increments in length. On account of the great precision of the new 
instrument such a device is needed, and accordingly the recording 
mechanism has been refined until it is capable of registering an 
increment of a single micron. 
It has hitherto been necessary, except with the auxanometer 
described by Frost," to place the recording device in close proximity 
to the plant. This is cumbersome and has prevented the simulta- 
neous recording of the growth of a number of plants. FRost’s 
machine used a thread, but as the growth was recorded electrically, 
simultaneous records of the growth of several plants could be made. 
This valuable feature has been incorporated in the new auxanometer. 
The machine consists essentially of a device which is carried 
upward as the plant grows. When this device has moved a certain 
small distance, it closes an electric circuit which operates the 
recording pen of a chronograph. As the connection of the plant 
with the circuit-closing device is made with invar, a metal with an 
exceedingly small coefficient of expansion with changes of tempera- 
ture, the growth can be accurately measured to a very few microns, 
and by using a condenser in the electric circuit, as described later, 
the growth can be recorded to a single micron. 
t Minnesota Botanical Studies no. 17. 1804. 
Botanical Gazette, vol. 53] 504 
