584 STATE BOARD OF AGRICULTURE. 



of au alternating current. Devices that accomplish this are usually called 

 rectifiers. Tliere are a number of different forms but the most satisfac- 

 tory one for this particular condition is the commutator. This is fastened 

 upon the shaft of the rotary converter. Before an experiment is made 

 this commutator is so adjusted as to eliminate one phase entirely. This 

 adjustment must be made accurately as the value of all the worli which 

 follows is dependent upon it. The brushes must also be kept clean. Care 

 in the above matters is necessary if the best results are to be obtained. 

 Fig. 14 shows the set-up of the apparatus when the commutator and 

 D. C. galvanometer are used. 



For this experiment a water of rather low conductivity was selected. 

 The resistance of this was determined by both the A. C. galvanometer 

 method and the device which has just been described. The comparison is 

 made from the standpoint of the range of scale divisions necessary for a 

 reading. 



A. Sol. Water of rather low conductivity, E. Bridge, T. 18° C, Known 

 resistance 51,100 ohms. Resistance in bridge circuit "out." Conduc- 

 tivity cup X, electrodes platinized and 2 mm. apart. 



Bridge Reading 



Method Left Middle Right Range 



A. C. Galv 500. 501.125 502.25 2.25 



D. C. Galv. and commutator 509. 509.4 509.75 .75 



This shows that the use of a D. C. galvanometer and commutator 

 shortens the range of scale divisions and makes the reading more precise. 



The condenser at hand, where this work was done, did not possess a low 

 enough capacity. If a suitable condenser had been at hand the range of 

 scale divisions would in all probability have been smaller. 



B. Sol. M/50 KCl, E. Bridge, T. 18° C, Known Res. 55 ohms. Res. in 

 bridge circuit unknown. A rheostat not graduated was used. Conduc- 

 tivity cup I, electrodes platinized and 5 mm. apart. 



No. 



The heating effect was completely eliminated by placing a variable resist- 

 ance in the bridge circuit. 



Experiment IV. 



This experiment is reported in order to demonstrate the presence of 

 heating effect, when the current load on the electrolyte is heavy. It also 

 shows how effectively it is eliminated by introducing a resistance in the 

 bridge circuit. 



The solution was M/lOO KCl, E. Bridge, T. 18° C, Known Res. 100 

 ohms, The conductivity cup with ground-in thermometer was similar to 

 that of figure 2 pictured on page 14, Leitvermogen der Elektrolyte, by 

 Kohlrausch and Holborn. The electrodes were platinized but the distance 

 between them was not determined. The form was that of a hollow cylin- 

 der. The bath thermometer was a standard one with Reichsanstalt cer- 



