10 



STUDIES OX FRUIT RESPIRATION. 



above the highest room temperature attained during the summer, 

 it was necessary to keep in mind the fact that such an incubator has 

 a small heat capacity and the heater must likewise be of low heat 

 capacity, so that the temperature may cease to rise as soon as the 

 circuit is opened; otherwise the temperature may rise and fall 

 through a wide range. 



The heating device, B, was a set of coils taken from an electric 

 toaster. This was placed directly in front of the fan, C, so that 

 the heat as supplied was quickly distributed through the chamber. 

 The thermostat, D, consisted of a long coiled glass tube filled with 

 mercury. It was closed at one end, mounted 

 on a wooden frame not shown, and provided at 

 the other with a simple device for making and 

 breaking a low-tension electric current as the 

 temperature varied. The essential part of the 

 thermostat is shown in detail in fig. 3. While 

 involving nothing new in principle, the arrange- 

 ment has been found to combine sensitiveness 

 with stability. Electric connection is made with 

 the mercury of the thermostat through the plat- 

 inum wire A. It is necessary that mercury of 

 high purity be used, that its surface be kept 

 free from dust, and that sparking at the point 

 of contact between the mercury and the plati- 

 num wire be reduced to a minimum. The heat 

 was supplied by the 110-volt, direct lighting cur- 

 rent which was cut in and out by a telegraph 

 relay, E, actuated by opening or closing the 

 small current at the point of contact in the 

 thermostat. 



Two points in the electrical arrangements 

 deserve special mention. First, sparking was 

 avoided in the usual manner by placing con- 

 densers G and H across the spark gaps at F and 

 I, respectively. In making and breaking the heating current it was 

 found that if the condenser was too large enough energy was stored 

 in it to cause small sparks when the circuit was closed at the con- 

 tact points, causing them to adhere at times so that they failed to 

 separate at the pull of the electro-magnet. The condenser finally 

 used for breaking this current consisted of three pairs of plates of 

 tin foil separated by waxed paper. The effective area of each sheet 

 was about 500 square centimeters. The second point is the method 

 of getting the small constant current which is made and broken in 

 the thermostat. This was obtained as a shunt from the 110-volt 

 direct current by using a resistance coil, J, rated at 1,000 ohms, con- 



Fig. 3.— Detail of ther- 

 mostat. 



