BRIDGMAN. — A SECONDARY RESISTANCE MERCURY GAUGE. 233 



will be discussed more in detail later; corrections due to air occluded 

 in the mercury ; and variations of elastic behavior of the glass envelope 

 under pressure. 



With the first few applications of pressure to the glass capillary 

 directly after drawing, the zero value of the mercury resistance under- 

 goes a permanent change, the magnitude of the change decreasing 

 with successive applications of pressure until finally after four or five 

 applications no further change is perceptible. This set is almost cer- 

 tainly due to a change of form of the glass vessel. This initial change 

 has been observed as large as 3 mm. of bridge wire, that is, y ^o of 

 the total resistance, and is always in the direction of decreased resist- 

 ance, that is, toward an increase of cross section of the glass, contrary 

 to what one might expect. If, however, this change of zero is caused 

 by a relieving of the internal strains in the glass, it is in the direction 

 one might expect, because the strains set up by drawing the capillary 

 down from a larger size might decrease upon increasing the size 

 toward its initial value. Not only is there zero change on the first 

 application of pressure, but the elastic behavior over the entire pres- 

 sure range, as shown by comparison with a well seasoned gauge, is 

 irregular. This irregularity of behavior is shown independently of 

 the resistance measurements by measurements of the compressibility 

 of the glass, which will be given in another paper. The remedy for 

 this defect is to season the glass by gradually applying and relieving 

 the pressure several times. Sudden changes in pressure, such as have 

 sometimes occurred when parts of the apparatus have exploded, are 

 accompanied by large changes in the glass. If the glass has been 

 subjected to considerable temperature changes after being seasoned 

 in this way, it must be seasoned again before its indications are 

 trustworthy. 



Occlusion of air in the mercury is likely to cause considerable 

 trouble if present in much quantity. Occluded air, as de Forest 

 Palmer remarks, was doubtless responsible for the surprisingly large 

 pressure coefficient of mercury resistance found by Lenz,3 0.0002. 

 The complete removal of the air is difficult and was accomplished only 

 once or twice. Boiling the mercury into the capillary several times is 

 a fairly efficient method, but is open to the objection, as suggested 

 above, that the glass must be seasoned again after each filling. Finally, 

 after several attempts, the following somewhat extravagant method 

 of procedure was found to work satisfactorily : One of the cups of the 

 U capillary was nearly closed by a glass stopper, and the whole U tube 



3 Lenz, Wied. Beibl., 6, 802 (1882). 



