334 BOTANICAL GAZETTE [NOVEMBER 
of the apparatus shown in fig. 1. This is a modification of a somewhat 
similar instrument secured through Dr. K. F. KELLERMAN, and used for 
some time at the Laboratory of Forest Pathology at Madison, Wisconsin, 
in controlling the temperature of a Bausch and Lomb incubator. 
The main advantage of the apparatus here described lies in the fact 
that the same current (110 volts, 60 cycle, alternating) which passes 
through the heating units is used, after reducing the voltage to 6, 8, or 
14 volts, to operate the relay (fig. 1D). The apparatus is simple, 
comparatively cheap, and when once set up and adjusted needs little 
attention. It will stand continuous service for long periods of time, and 
when used in connection with a toluol and mercury filled tube (fig. 1G) 
controls temperatures within a range of a quarter of a degree Fahrenheit. 
When the mercury column in the tube (G) rises and contacts with 
the fine wire held by the cork, the coils at D draw the armature bar up 
to the poles of the magnet, the U-shaped wire is pulled free from the 
mercury at E, and the current supplying the heating unit is broken, thus 
shutting off the heat. A reversal of this action turns the heat on again. 
At E the make and break of the current supplying the heat units is 
very positive in action. There is no arcing of the current between the 
mercury and the wire points, so long as the U-shaped wire is raised 
sufficiently above the mercury surface. The small voltage of the current 
passing through the mercury column in the tube (G) reduces the spark 
to a minimum, and for this reason little trouble is encountered with 
arcing, vaporizing of mercury, or clouding of the tube where contact 
is made between the fine wire and the mercury. A metal cap with 
adjusting screw carrying a fine platinum wire may be substituted for 
the cork and wire as shown.—Ernest E. Hupert, Laboratory of Forest 
Pathology, Bureau of Plant Industry, in cooperation with the Forest Products 
Laboratory, Madison, Wisconsin. 
