320 
Journal of Agricultural Research 
Vol. V, No. 8 
mechanical strain and surrounding it by a small dead-air space which 
serves to some extent as a heat insulator, protecting it from changes in 
temperature of the laboratory air. The side openings in the fittings 
provide an inlet and an outlet for the water. 
The electric heater, which is in the lower end of the channel, consists 
of platinum wire, of 55 ohms’ resistance, in a flat coil about 10 cm. long 
and 9 mm. wide, inclosed in a flat case of thin metal which, with the coil 
inside, is 10 mm. wide and 2 mm. thick. At one end this flat part of the 
case tapers into a tube about 3.5 cm. in length and 6 mm. in diameter, 
in which are the wires carrying electric current to the coil. This heater 
is inserted in the water channel, through the open end of the fitting, 
to the depth at which the whole of the resistance wire will be immersed 
in the water current, and a packing device in the end of the fitting is 
tightened around the neck of the case to hold the heater in place. In the 
channel the heater is surrounded by a space 1 mm. across, through which 
the water flows. Heat generated in the coil is imparted instantly to the 
water which surrounds the heater in such a thin layer that the temperature 
of the whole mass of water is very quickly affected. 
The electric current flowing in the heating coil is determined by ballast 
resistance in series with the coil, of which 125 ohms are fixed and 550 ohms 
variable. For the former an ordinary resistance unit is satisfactory, and 
for the latter a rheostat of oxidized constantan wire of graduated cross 
section wound on an insulated light-steel tube has given excellent service. 
The sliding contact on the rheostat may be moved by hand or by means 
of a screw shaft (PI. XXXV, fig. 2). When the total resistance of both 
the rheostat and the resistance unit is in series with the coil, a current of 
approximately 0.3 ampere will flow in the coil, the heating effect of which 
is sufficient to increase by a little less than 0.1 degree the temperature 
of water flowing through the heater at the rate of 1 liter per minute. 
When the whole of the rheostat is out of the circuit and only the resistance 
unit is in series with the coil, the current will be approximately 1.2 
amperes, with a heating effect sufficient to increase by a little over 
1 degree the temperature of 1 liter of water per minute. 
Between these limits the heating effect may be varied in large or small 
steps, according to the distance the sliding contact is moved along the 
turns of wire in the rheostat. If the position of the slider is adjusted 
by hand, any portion of the rheostat, from the total resistance to that of 
a single turn of the wire, may be instantly put into or out of the circuit. 
That the temperature of the water may be automatically regulated, 
however, the position of the slider is adjusted by a screw shaft. A small 
pulley on the end of the shaft is belted to another pulley on the arma¬ 
ture shaft of a small electric motor, that may be caused to run in one 
direction or the other and for a longer or shorter period, depending upon 
whether the amount of resistance in the circuit must be increased or 
decreased and how much. The field coils of the motor are differentially 
