OF SALTS IN ETHYL ALCOHOL AND WATER. 83 



this system very thoroughly to be sure that all external resistance in 

 the circuit was negligible, or, when it was not, we determined its exact 

 magnitude in order to make the proper corrections. 



The system of wiring was a double one, so that by means of a 

 double-throw, double-blade switch, both a and b in the formula for 

 calculating conductivity 



va 



could be read directly on the bridge. If two standard resistance boxes 

 are connected one to each side of the bridge, and plugs representing 

 equal resistances are removed from both boxes, then the reading of the 

 wire will be 500 mm., or exactly its middle-point; provided, of course, 

 that there is no appreciable resistance in the circuit itself (if there is it 

 must be evenly balanced). Such was the case with our equipment. 

 Therefore, there were no corrections of this kind to be made to a or b 

 in the conductivity determinations. 



But this double system also serves another purpose. When the con- 

 ductivity of a solution is being measured, if both a and 6 are read on the 

 bridge wire for the same resistance supposing that all other conditions 

 remain constant then the mean of the two readings will be 500 mm. 

 If we do not find by actual experiment that our mean value is 500 mm., 

 we may be sure that at least one of the other conditions, such as, for 

 example, temperature, is varying and needs attention. 



It was very difficult in some of the measurements to obtain distinct 

 minima. The distances covered on the wire on either side of the 

 minimum point in such instances, were so great before finding corre- 

 sponding sounds on the two sides that the minimum point itself could 

 be only approximated. We endeavored to overcome this difficulty by 

 connecting a condenser in parallel with the rheostat. In determining 

 the conductivities of the alcohol and alcohol-water solutions, it was 

 practically useless; the fact is, the condenser actually made the read- 

 ings in some cases harder to obtain. However, in determining the 

 cell constants of those cells which required low resistances, the bridge 

 readings were made much sharper over a shorter distance, and the 

 minimum very much more distinct by the use of the condenser. We 

 did not arrive at any general conclusion concerning its use. 



TEMPERATURE REGULATORS. 



A number of different types of thermo-regulators have been used in 

 this laboratory from time to time; of the earlier forms it is not necessary 

 to speak. The regulators used during the last two or three years have 

 had the general form of that shown in figure 31, and were filled with 

 mercury. 



At the beginning of this investigation we devised a regulator (fig. 32) 

 in which only the trap bulbs and capillary contained mercury, the series 



