40 
GEORGE ,F. POTTER 
chamber passes through an opening in the exact center of the insulated 
cover, which is therefore free to revolve about the pipe. Ice and salt may 
be placed in any part of the two outer spaces (a and h, Plate I), through an 
opening near the outer edge of this cover. 
In such a chamber it is always difficult to keep the temperature uniform 
from top to bottom. To accomplish this as nearly as possible, a false wall 
or tin cylinder about two inches smaller in diameter than the freezing 
chamber, is introduced and held in place with wooden blocks (longitudinal 
section of freezer, Plate I). The cold wall of the chamber outside and the 
heating coil within cause the air to circulate upward inside the false wall 
and downward outside it. In addition, the circulation is forced by a fan 
at the bottom of the chamber. When one is freezing parts of plants that 
do not obstruct the passage of air, thermometer readings indicate that there 
is less than o.i° C. difference in temperature over a vertical distance of 
eight inches. 
The heating coil is wound with about 30 feet of no. 30 "Chromel C" 
resistance wire, having a total resistance of about 200 ohms. As a rule a 
large low-resistance lamp, or two 40-watt lamps in parallel, are placed in 
series with the coil to reduce the amount of heat given off and to act as a 
pilot light. On a iio-volt A. C. lighting circuit about one half ampere or 
less of current is used. The lamp can be switched out of the circuit if 
more heat is needed, as for instance when a large quantity of freezing mix- 
ture has just been added. An ordinary Dim-a-lite " or "Hylo" connec- 
tion is also placed in the circuit and can be used to reduce the amount of 
heat given off by the coil, as for instance when the ice is nearly exhausted. 
These adjustments are frequently convenient, although not necessary for 
the operation of the machine. 
A mercury thermostat (C, Plate II) controls the heating coil by means of 
a telegraphic relay operating on current from two dry cells. When the 
machine is operated continuously it is necessary to have two batteries of 
two cells each. One battery may then recuperate while the other is in use. 
The connection is conveniently alternated by means of a double throw 
switch. The thermostat consists of a piece of capillary tubing sealed to a 
bulb containing mercury. An enlargement at the top of the capillary 
holds the excess mercury at temperatures above the working range. The 
thermostat used by the writer has a bulb about i cm. in diamieter by 13 cm. 
in length and contains sufficient mercury to cause the mercury in the 
capillary to rise or fall about 2 mm. for each degree Centigrade change of 
temperature. Electric connection is made between the mercury in the 
bulb and that in a side arm by means of a platinum wire sealed in the side of 
the bulb (C, Plate II). A platinum wire guided to the exact center of the 
capillary by a small glass rod makes contact at the top of the mercury 
column {CC, Plate II). The guide is indispensable for accurate results 
because it makes an appreciable difference in temperature whether contact is 
