442 Electrkal Baths for Urea Determination [Mar. 



The current is turned into the constant temperature bath at the 

 same time that it is directed into the preliminary drying bath. The 

 double throw switch is turned to the bell side, the two heating coils 

 in the bath are thereby connected in parallel and rapid heating of 

 the bath ensues. When the switch is in this position the 154° C. 

 mercury regulator is connected with the bell and the resistance slab. 

 As soon as the bath reaches this temperature, the bell rings. The 

 switch is then thrown to the relay side. 



Two mercury regulators are necessary. With only one regula- 

 tor, the excess of heat in the heating coils sends the bath about 10 

 degrees above the indicated point, if the switch is thrown when the 

 bath has reached the desired temperature. Therefore two regu- 

 lators are used, one set about 10 degrees below the desired tempera- 

 ture. If the switch is thrown when 154° C. is attained, the excess 

 of heat in the coils sends the temperature of the bath up to the de- 

 sired point (164° C). When the switch is thrown on the relay 

 side, the one coil in the bath is in series with the resistance on the 

 rheostat. The rheostat has attached to its sliding arm, an insulated 

 arm resting on the next lower resistance. This insulated arm is 

 connected through a contact in the relay with the sliding arm. 

 The relay, resistance slab and 164° C. mercury regulator are con- 

 nected through the relay side of the switch. When the temperature 

 is below 164°, the contact in the relay remains unbroken. When 

 this temperature is reached the current flows through the relay and 

 the connection between the insulated arm with the sliding arm is 

 broken. 



It is necessary to ascertain experimentally, where to set the 

 sliding arm on the rheostat so that a little more current than neces- 

 sary is supplied to the bath when the relay contact is made, and that 

 a little less current than necessary is supplied to the bath when the 

 relay contact is broken. On our apparatus, when in use, the slid- 

 ing arm of the rheostat rests on point No. 8 and the insulated arm 

 on point No. 7. The resistance slab facilitates the use of "house 

 current " for the bell and relay, and obviates the necessity of 

 employing less reliable battery currents. 



The bath maintains great regularity of temperature. In six 

 hour runs it varies less than half a degree. The preliminary desic- 



