BRIDGMAN. — A SECONDARY MERCURY RESISTANCE GAUGE. 231 



of liquid is so speedily conducted away by the metal that one has to 

 work with inconvenient rapidity after increasing the pressure to find 

 any trace of this effect. This seems to dispose of the only real advantage 

 claimed by Lisell for the solid metallic resistance over the mercury gauge. 



The electrodes are of platinum, one soldered to the outside shell of 

 the plug, and the other to the inner stem, which is insulated from con- 

 tact with the liquid by a layer of marine glue. The electrode leading 

 from this stem is insulated with a soft fine rubber tube, except where 

 it enters the cup of the capillary, where it is covered with a piece of 

 glass tubing, joined continuously to the rubber above it with gutta 

 percha. The electrode from the outer shell of the plug is also pro- 

 tected with glass where it enters the other glass cup. This precaution 

 showed itself necessary, for otherwise if the platinum is not kept from 

 contact with the walls of the cup the liquid above shows an appreciable 

 tendency, with the successive lowerings and raisings of the surface by 

 each application of pressure, to creep down the glass past the mercury. 



There are several sources of error here that must be guarded against. 

 Possible short-circuiting from one electrode to the other through the 

 liquid has already been excluded by the measurements of the insula- 

 tion resistance of the plug with a dummy capillary. In addition, the 

 resistance of the electrodes between the mercury and the plug may 

 change because of (1) lengthening of the free part of the electrode by 

 depression of the mercury surface under pressure or distortions in 

 the containing vessel, (2) pressure effects on the specific resistance of 

 the platinum, (3) and change in resistance at the soldered connection 

 between the electrodes and the plug. The first two sources of error 

 may evidently be eliminated by using heavy enough electrodes. In 

 this work electrodes 0.8 mm. in diameter were large enough. The 

 third effect was found to be troublesome by Lisell, who avoided it by 

 using long metal wires of resistance high in comparison with the re- 

 sistance of the joint. No trace of this effect could be found, however, 

 in this investigation. The absence of all three effects was tested by 

 measuring the resistance when the terminals were short-circuited by 

 dipping into a large tube of mercury, the resistance of the mercury 

 now being negligible. In this case, the depression of the mercury due 

 to compression is much greater than in the U capillary actually used. 

 In the form tried, this depression may amount to 0.2 mm. Measure- 

 ments were made up to 7000 kgm., and no change in resistance of the 

 platinum terminal occurred of so much as js l ors ohm, the smallest 

 quantity that could be detected on the bridge. The possible error 

 here, therefore, when the resistance to be measured is 10 ohms, is less 

 than one part in 15000. 



