636 



SCIENCE 



[N. S. Vol. XXVII. No. 694 



form shown in Fig. 1 has proved so very 

 useful in the laboratory and also that several 

 teachers who have seen the apparatus have 

 desired a duplicate for their work. And there 

 may be others who would iind such a cell as 

 useful as this one has been. 



The principal object in the design of this 

 cell is to present the concept of " molecular 

 conductivity " of an electrolyte in such a clear 

 and concise form that no one who is capable 

 of making electrical measurements can mis- 

 understand it. The apparatus consists of a 

 strong glass tube provided with a small side 

 tube for filling. Both ends are ground plane 

 and closed by platinum-faced electrodes, the 

 whole being firmly clamped together in a suit- 

 able framework. Good insulation between the 

 electrodes is provided by making a portion of 

 this framework of ebonite, as shown in the 

 figure. Crushing strains due to the unequal 

 contraction of glass and metal are avoided by 





Glass O tube. 



m 



the use of heavy rubber washers, one on each 

 side, which take up the extra length without 

 much increase in pressure. It has been found 

 that a moderate pressure is suificient to pre- 

 vent leakage between the ends of the glass 

 tube and the mental electrodes. The appa- 

 ratus is supported by four short legs not 

 shown in the figure. 



Th6 resistance of the column of liquid con- 

 tained in this tube is measured by the method 

 of Wheatstone's bridge, using a telephone and 

 alternating current. The specific resistance, 

 s, of such a conductor has the same meaning 

 as for a metallic wire and is given by the 

 same relationship, 



s = r A/L, 



where A is the cross-section and L the length 

 of the column whose resistance is r ohms. 

 Both A and L can be measured directly, and 



each has a very definite and unmistakable 

 meaning which is readily understood by the 

 student. This can not be said regarding the 

 equivalent " constant " of a cell which has no 

 definite dimensions. 



The specific conductivity, c, or the con- 

 ductivity of a centimeter cube of the solution 

 from one face to the opposite one, follows at 

 once as the reciprocal of this. Or 



c= l/s = L/rA. 



Since the conductivity of an electrolyte de- 

 pends upon the amount of the substance in 

 solution — that is, upon the number of ions 

 per cubic centimeter — if the conductivities of 

 different solutions are to be compared it is 

 necessary to express the concentrations in 

 comparable terms. This is usually done by 

 stating the number of gram molecules, m, 

 of substance that are dissolved in each cubic 

 centimeter of the solution. The molecular 

 conductivity, ft, of an electrolyte is then 

 defined as the conductivity of a centimeter 

 cube of the solution per gram molecule of salt 

 within this cube. In symbols, 



fi = o/m. 



Presented thus there is rarely a student who 

 does not understand from the fiLrst the mean- 

 ing and significance of the molecular conduct- 

 ivity of an electrolyte. No claim is made 

 that this apparatus is specially adapted for 

 refined research or that it is the best form for 

 the expert. For example, it can not well be 

 placed in a constant temperature bath as is 

 necessary for all refined measurements. But 

 nevertheless when working at room tempera- 

 tures, and using solutions which have stood in 

 the same room for some time, very satisfactory 

 results can be obtained, and for the purpose 

 for which it was designed this apparatus has 

 met every requirement. 



Arthur Whitmore Smith 



UNr^EESITT OF iMiCHIGAN 



THE AMERICAN ASSOCIATION OF 



MUSEUMS 



The American Association of Museums will 



hold its third annual meeting in Chicago, 



May 5-Y, as the guest of the Chicago Art 



