APPARATUS FOR CHANGING TEMPERATURE OF A CHAMBER 4I 
made at the center or at the edge of the convex meniscus. An insulated 
wire is run from the battery to the side arm. The metal parts of the appara- 
tus acting as a ground wire carry the current to the contact in the capillary. 
A condenser is used to prevent sparks at the point of contact with the 
mercury, and to eliminate the arc completely it is necessary to short cir- 
cuit the terminals of the condenser through a resistance of about 700 ohms. 
A fifteen-watt Mazda bulb is used for this purpose. 
The point at which the platinum wire touches the mercury in the 
capillary tube controls the temperature at which the heating coil is brought 
into use, and hence controls the temperature of the chamber. The con- 
tact wire is attached to a plunger, which moves vertically in guides fastened 
to the T-bar above the thermostat (longitudinal section, Plate I, and B and 
C, Plate II). The mechanism which actuates this plunger is similar in 
principle to a recording thermometer (E, F, and G, Plate I). The hands 
were removed from a'' Big Ben" alarm clock and a shaft, bearing a drum 
three inches in diameter and four inches long, was soldered to the hour- 
hand shaft. A time-temperature chart is attached to the drum, degrees 
being marked by equal spaces along the axis of the cylinder, and hours by 
twelve equal spaces about its circumference. The distance to be laid off 
for each degree depends on the sensitivity of the thermostat and on the 
relative lengths of the two levers which will be referred to as the "long arm" 
and "short arm" of the ''revolving shaft." The time-temperature curve is 
constructed with a flexible lead bar, fastened at each end by a clamp (e) 
sliding in a groove which runs lengthwise of the drum (F and G, Plate I). 
Below the drum and at right angles to it there is a revolving shaft (E, Plate 
I). At the clockwork end of this shaft a long arm, somewhat similar to 
that which carries the recording pen of the thermograph, extends upward and 
engages the lead bar with a small connecting pin (E and G, Plate I). At the 
other end of the shaft, which is pivoted in a bearing at the top of the T-bar 
leading down into the freezing chamber, there is a short arm which is 
joined by means of a connecting rod to the plunger carrying the platinum 
contact wire of the thermostat (A and B, Plate II). The weight of the 
connecting rod and plunger acting on the short arm as a lever tends to 
revolve the shaft and thus keeps the connecting pin of the long arm in con- 
tact with the edge of the lead bar. As the drum revolves, a movement of the 
long arm is permitted in accordance with the curve traced by the lead bar, 
and a proportionate movement is transmitted to the plunger and contact 
point of the thermostat through the turning of the shaft and through the 
resulting movement of the short arm and connecting rod. The short arm is 
attached to the shaft in such a way that it forms a right angle with the 
connecting rod when the long arm is at right angles to the axis of the drum. 
The vertical movement of the plunger is therefore always exactly propor- 
tionate to the horizontal distance which the long arm moves along the axis 
of the drum, although the amount that the shaft revolves for each degree of 
temperature is greater as the arm approaches either end of the drum. 
