320 Journal of Agricultural Research voi. v. no. s 



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 i 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 1 25 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 ser^dce. 

 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 Httle less than o.i degree the temperature 

 of water flowing through the heater at the rate of i 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 Uttle over 

 I degree the temperature of i liter of water per minute. 



Between these limits the heating effect may be varied in large or small 

 steps, according to the distance the sUding 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 



