566 Mr. F. E. Smith on Bridge Methods for Resistance 



The Dial Contacts. — For precision work it is necessary 

 for the dial contact resistances to be fairly reproducible, and 

 this involves well-designed brushes and good workmanship. 

 Before attempting to design a dial-form bridge we made 

 some experiments on the constancy of the contact resistances 

 in a Wolff potentiometer, and satisfied ourselves that the 

 contact resistances in six well-made dials in series will keep 

 constant within about 0*0002 ohm over a period of twelve 

 months. Mr. Whipple, of the Cambridge Scientific Instru- 

 ment Co., has very kindly given us values approximating to 

 the contact resistance of a special form of brush made by 

 them. The resistance was measured from below the stud of a 

 dial, through the brush contact to a position on the brush 

 ring. The contact resistance is therefore less than any of the 

 values cited. With vaseline on the contacts the resistances 

 were found to remain very constant, and on similar studs 

 the values measured were : — 



No. 1 0-000289 ohm. 



„ 1 0*000280 „ 



„ 2 0-000288 „ 



„ 2 0-000289 „ 



Tn each case the second reading was taken after rubbing 

 over the stud ten times. These results are most gratifying. 

 In the 6-dial bridge (fig. 12) it would be necessary for a 

 variation of 0*001 ohm (i. e. nearly four times a single contact 

 resistance) to result to produce an error equivalent to 

 0°-001 C. 



The fact that the adjustable resistances are arranged in a 

 dial form renders the bridge particularly useful where rapid 

 observations are necessary. The galvanometer circuit is 

 permanently closed, so that thermal e.m.f's produce a 

 minimum of disturbance, and the sensitiveness of the bridge 

 is practically as great as any combination of resistances 

 hitherto used for the measurement of temperature by means 

 of a platinum thermometer. For general use a galvanometer 

 of from 5 to 20 ohms resistance may be used. 



Fourth Method. 



In this the leads of the platinum thermometer are practi- 

 cally eliminated by shunting two of them with suitable 

 resistances after the manner of the Kelvin double bridge. 

 The connexions are shown in fig. 15. 



