570 STATE BOARD OP AGRICULTURE. 



water" or a solution wliich is extremely dilute in respect to its solute 

 content. Conversely it follows that the electrodes must be far apart and 

 the surface small, if one wif^hes to measure the resistance of concentrated 

 solutions. In the former case, conditions are such as to favor the passage 

 of a strong current, while in the latter a weaker current is allowed to 

 pass and a further hindrance to its passage is etlected by increasing the 

 distance through which it is required to pass. 



It is necessary that the platinum be of the best quality and free from 

 lead and calcium. Also it must be stiff enough so as not to change shape 

 or get out of position. Otherwise the cell constant would vary and intro- 

 duce a source of error. Certain modifications of the cell and its elec- 

 trodes have much to do with the control of error from polarization. 

 Platinized electrodes increase the surface enormously. Kohlrausch esti- 

 mated that a coating of platinum black increased the surface a thousand 

 times. With electrodes of such large surfaces polarization is negligible. 

 Platinum black absorbs the gas as soon as it is separated out and it does 

 not therefore modify the concentration of the solution. The statement 

 has been made that it is unnecessary to platinize electrodes. From our 

 experience with a frequency of 60 cycles per second we obtain better re- 

 sults with platinized electrodes than with bright electrodes. It has been 

 argued that bright electrodes are preferable since platinized electrodes 

 cannot be used in all cases. l*latinum black acts as a catalytic agent 

 under certain conditions and will bring about chemical reactions. A 

 proper selection of the solution or a modification of the electrodes will 

 obviate this difficulty. Again, on account of the spongy nature of plati- 

 num black, it is argued that it will absorb some of the electrolyte, and 

 when the same cell is used for another solution of different concentration 

 or character, it will cause an error in the measurement. By taking a 

 new clean cell, or by cleaning the old one this trouble can be corrected. 



Another factor in regard to the construction of resistance cells must be 

 kept in mind. There must be the smallest amount of space possible 

 between the lateral edge of the electrodes and the glass walls of the elec- 

 trolytic cup. When working with nutrient culture solutions in different 

 sized beakers a new form of electrolytic cells is used. This is called the 

 "plunger" cell. The electrodes are enclosed in a cylindrical glass chamber 

 and always maintain the same position in reference to the side walls of 

 the chamber. The chamber is ])ierced at the bottom and top by three 

 holes. When this is plunged into the liquid in the beakers the solution 

 passes through the holes and surrounds the electrodes. When everything 

 is at rest a measurement can be made. When measuring the resistance 

 of living or dead tissue another form is required. This must be made 

 mechanically rigid and allow for an arrangement whereby the tissues 

 can be securely and firmly clamped between the electrodes. This form is 

 one devised particularly for work with tissues and is figured in this 

 paper. 



SUMMARY. 



The following conclusions can be drawn from a consideration of what 

 has been said concerning the sources of error in the usual Wheatstone 

 Bridge set up. The induction coil as a source of current must be aban- 

 doned. The telei)houe is not as sensitive as the alternating current gal- 

 vanometer of the dynamometer type. The common resistance coils are 



