286 MR. W. R. BOUSFIELD AND DR. T. M. LOWRY ON THE ELECTRICAL 







and from measurements of a 45-per cent, solution O'Ol per cent, higher than the 

 value given above. 



(4.) The Measurements of Conductivity. The resistances of the solutions were 

 measured by using an alternating current and a telephone. The wheel-bridge 

 recommended by KOHLRAUSCH (KOHLRAUSCH and HOLBOEN, ' Leitvermogen der 

 Elektrolyte,' p. 42) was used and found to be both convenient and accurate. The 

 main disadvantage of the bridge is that the sensitiveness decreases considerably 

 when used in comparing unequal resistances, and that the bridge-wire must be 

 carefully calibrated at intervals of a few months if the instrument is in constant use. 

 These difficulties were overcome by using the comparison coils (1, 10, 100, 1000 

 and 10,000 ohms) supplied with the bridge only in determining the approximate 

 resistance of the electrolyte, whilst the final measurements were made by comparison 

 with an equal resistance. The resistance-box used for this purpose was specially 

 wound so as to be almost free from electrostatic capacity. The exact position of 

 silence on the bridge-wire was determined with three resistances differing slightly 

 from one another, and in this way three readings were obtained, from each of which it 

 was possible to calculate the resistance. As the readings were all close to the centre 

 of the wire, a maximum of sensitiveness was obtained, and it was only necessary to 

 check from time to time the exact position of the electrical centre of the bridge-wire. 

 The three readings were usually concordant within one or two parts in 10,000, and if 

 necessary were repeated until concordant results were obtained. The method may 

 be illustrated by an example taken from a recent series of measurements. 



Resistance Reading of Resistance 



of coils. bridge-wire. (ohms). 



140-5 500-2 140-61 



140-4 500-3 140-57 



140-6 500-0 140-60 



Mean . . . 140-59 

 Leads 0-10 



140-49 



Resistance capacity 13-738 cm." 1 ; specific conductivity K 18 = 0-09775 cm. ohm" 1 . 



Concordant readings of the instrument do not necessarily indicate a corresponding 

 approach to accuracy in the values deduced for the conductivity of the electrolyte. 

 The errors due to heating, polarisation, self-induction, and capacity are fully discussed 

 by KOHLRAUSCH and HOLBORN in the volume already referred to (pp. 54-59). Of 

 these the most serious are those due to polarisation when the resistance is low, and to 

 electrostatic capacity when the resistance is high. In our experience the most favour- 

 able resistance to measure is about 50 ohms to 100 ohms, but in determining the 

 conductivity of a solution over a range of temperature involving large changes in 

 conductivity we have occasionally been obliged to measure resistances as high as 



