BRIDGMAN. — A SECONDARY MERCURY RESISTANCE GAUGE. 229 



reaching a steady value which is of the order of 100,000 ohms. The 

 lowest resistance found in any of these plugs was 30,000 ohms. The 

 resistance of these plugs was measured under pressure, all the condi- 

 tions of the actual experiment as to position of the electrodes, etc., 

 being reproduced, except for a dummy glass capillary to hold the 

 mercury. When in use, the insulation resistance sometimes increased 

 under pressure, the increase being sometimes as much as 100 per cent. 

 This is still outside the limits of error, the error introduced in the 

 above most unfavorable case being only one part in 6000 on the 

 apparent resistance of the mercury. The performance was usually 

 much better than this. Thus the insulation resistance of one plug 

 which seemed to settle down after several applications of pressure 

 at 150,000 ohms was found to be 220,000 after seven more applica- 

 tions of 7000 kgm. 



In devising a form of vessel for holding the mercury, endeavor was 

 made to keep the mercury as much as possible from contact with all 

 sources of contamination by the use of platinum electrodes and a 

 containing vessel entirely of glass. Other 

 experimenters have allowed the mercury 

 to come in contact with the steel of the 

 containing vessel, using the vessel as one 

 electrode, but this seems undesirable in 

 view of the somewhat large effect of mi- 

 nute quantities of impurity. Many forms 

 of glass containing vessel which readily 

 suggest themselves are impractical because 

 of the impossibility of using platinum 

 electrodes sealed into the glass, the differ- 

 ence of compressibility between platinum 

 and glass being sufficiently great to crack 

 the glass around the electrodes. Two 

 forms were finally adopted and used. 

 The form first used was a U capillary 

 (Figure 5), the electrodes dipping into the 

 two cups at the upper end. In the form 

 originally used this was made of ther- 

 mometer tube of about 6 mm. outside 

 diameter and 0.1 mm. bore. Several 



times, however, even when carefully an- resistance is to be measured, 

 .nealed, the glass cracked at the bend, If the glass is too thick, it in- 

 apparently because of the unequal strains variabJ y breaks under pres- 

 set up by the hydrostatic pressure within sure at the bend A> 



V 



v 



Figure 5. Original and 

 final form of the receptacle 

 for holding the mercury whose 



