580 STATE BOARD OF AGRICULTURE. 



pared by iisiug a cork-borer or other suitable device. Tlie.se discs are then 

 piled oue upon another like a pile of pennies and clamped firmly between 

 the electrodes. The i)ipotte forms are similar to those figured in a paper 

 by Washburn and Machines (43). It is preferable to have the electrodes 

 vertical rather than horizontal for the quick removal of heat. It is ad- 

 visable to immerse the bridge leads in tubes containing mercury, these 

 tubes to be immersed in the bath so as to be at the same temperature as 

 the cell. This reduces the passage of heat between the room and the cell. 



THERMOSTAT. 



The conductivity of a solution is greatly affected by temperature 

 changes. A variation of one degree gives an error of 2% in the readings. 

 It is necessary therefore to keep the temperature constant during con- 

 ductivity measurements. For accurate measurements, a variation of .01 

 of a degree C. is not permissible. The best thermostat contains a large 

 body of water to reduce the effect of temperature changes of the sur- 

 rounding bodies. This water must be kept in constant motion and at a 

 constant temperature. 



Various devices for automatically regulating temperature have been 

 tried. The really efficient one is electrical. The electrolytic cells are 

 suspended in the bath and allowed to remain so for at least 10 minutes 

 before a measurement is taken. Then the resistance, that is to balance 

 the solution to be tested is adjusted, approximately. The bridge wire can 

 be used alone or in combination with the "extensions." For accurate 

 work the "extended wire" must be used. A modification of the Ostwald 

 thermostat was used for preliminary work. This only kept the tempera- 

 ture constant to within a .1 of a degree. For accurate work the Freas 

 water thermostat may be used. In this one the temperature is kept con- 

 stant to .002 of a degree. A Beckmann thermometer should be suspended 

 in any water bath as a check on the thermostat. The temperature of the 

 bath was kept constant at 18° C. 



4. METHOD OF OPERATION. 



The following method of operation is suggested. See that the bath 

 temperature is correct and constant. Select the cell that will be best 

 suited for measuring the solution in question. Fill the cell and suspend 

 it in the thermostat. When the solution is at the temperature of the bath 

 a measurement may be made. Now start the rotary converter, being 

 sure that the double contact switch which closes the bridge and galvan- 

 ometer circuits is open. Throw in 500 or 1,000 ohms resistance in the 

 variable rheostat so as to cut down the current entering the bridge. This 

 will prevent excessive deflection of the galvanometer scale. Observe the 

 zero point on the galvanometer scale and then close the galvanometer and 

 bridge circuit switch for an instant. Note the direction of deflection. 

 This will indicate whether the known resistance is too high or too low. 

 Adjust the resistance to suit condition and close switch again. Repeat 

 operation until balance is approximately obtained. Now vary the cur- 



