564 STATE BOARD OF AGRICULTURE. 



is the Vreeland Oscillator recently put on the market. It is the easiest 

 to keep in running order and most convenient to handle. No condensers, 

 frequency meters, hot wire ammeters, etc., are necessary. It is noiseless 

 and therefore can be kept near the operator if need be. In addition, it 

 has the great advantage of yjroducing a pure sine wave. Even in the 

 cheaper form, the price of this instrument is $225, and is obviously too 

 much to lay out on one part of a Wheatstone Bridge network. 



All these instruments give a frequenc.y of 1,000 cycles per second. The 

 u.se of a frequencj' of GO cycles in conductance measurements of liquids 

 will, in all probability, be viewed witli suspicion, and we might also add 

 that that was our earlier attitude. It is asserted that, working with such 

 low frequencies, the results are liable to error from polarization. How- 

 ever, after the continued use of this frequency, in connection with a sen- 

 sitive indicator and the control of certain other necessary conditions, 

 over a period of two years, we believe that the difQculties assumed are 

 more apparent than real. Let us consider the theory for a moment. We 

 can assume that, for a very small current passing for a short time, the 

 e. m. f. of polarization is proportional to the quantity of electricity which 

 has passed through the cell. When measured by a balanced Wheatstone 

 Bridge the resistance will increase with the time that the current is kept 

 on and will always be higher than the true resistance. For this reason 

 the use of the Wheatstone Bridge with direct current is not suited to the 

 determination of the resistance of solutions of electrolytes. If, on the 

 other hand, one uses alternating current and a sensitive detector the 

 principle of the Wheatstone Bridge can be used with accuracy. This is 

 so, quoting from Dr. Northrup (17), "because the e. m. f. of polarization), 

 produced by the current in one direction, and which would lead to a 

 balancing of the bridge giving too high a value of the resistance, will, 

 upon the reversal of the current, either be neutralized or, if not neutral- 

 ized, will lead to a balancing of the bridge giving too. low a value of the 

 resistance. Thus the setting actually obtained for a balance is the same 

 whether polarization is neutralized or not, as would be required were 

 there no polarization." A statement of this kind from such an authority 

 should cause us to reconsider our ideas of the possible error from 

 polarization. 



It is a well known fact that the amount of hydrogen separated per 

 square centimeter at the electrode is directly proportional to the current 

 passing. By regulating the current so as to make it small, the amount of 

 substance separated at the electrodes, or in other words the polarization 

 can be made negligible. When the alternating current changes rapidly 

 such as it will at a freauency of a thousand we are told that polarization 

 is not likely to take place. There is not sufficient time for hydrogen to 

 separate out before the phase is changed. At GO cycles where more time 

 elapses before the change to the other phase takes place it has been 

 assumed that the hydrogen might possibly have time to diffuse into the 

 solution and thus change the concentration. We are told that only under 

 special conditions will the error from polarization be rendered small 

 enough to permit measurements at 60 cycles per second. These state- 

 ments above might lead one to believe that we have taken a lot for granted 

 and accepted without reserve a lot of assumptions. In all our experi- 

 ments we have not had any trouble from polarization which could not be 

 eliminated. Because of the nature of the indicator we use it is not neces- 



