116 



On a new Form of Resistance-Balance. 



up a little, and thus a short length of mercury interposed, 

 which leads to an error in a reading. 



5. Example of a determination of the Variation-coefficient of 

 a Coil. — The whole resistance of the platinum-iridium wire is 

 very nearly 0*0512 of an ohm, or not far from ■£§ of an ohm, 

 at about 15° Cent. As the whole length can be divided by 

 the vernier into 10,000 parts, this gives as the value of -^q of 

 a division ^q^o oo °f an onm - 



The unit in the following example is t 'q of a division. To 

 secure the greatest accuracy of measurements a low-resistance 

 galvanometer must be used. I am in the habit of using one 

 having a resistance of about half an ohm. The image of a wire 

 strained across a slit is reflected on a scale in the usual way, 

 and read at a distance by means of a telescope. This galva- 

 nometer will give an indication, when used with precautions, 

 due to a difference of one tenth of a division when comparing 

 two ohm coils. But as the temperature can hardly be mea- 

 sured with certainty to within less than ^ of a degree, this 

 alone renders such refinement of reading nugatory, in the ab- 

 sence of better methods of ascertaining with certainty the real 

 temperature of the wire. 



Two coils were compared. Call them F and K, K is the 

 coil whose variation-coefficient is required. 



I. Difference of Resistance of Coils F and K at 11° Cent. 





Bridge-readings. 



Difference. 



Exp. i 



Exp. ii 



Exp. iii 



5000 

 5000 

 5000 



4955 

 4954 

 4955 



45 

 46 

 45 



The first column gives the number of experiment, the second 

 the reading with the coils F and K in one position on the 

 bridge, the third when F and K are reversed or have exchanged 

 places on the balance ; and the fourth gives the difference of 

 their resistances at 11° C. in units of the bridge-wire. 



II. Difference of Resistance of Coils F and K at 28°*2 Cent. 





Bridge-readinge. 



Difference. 



Exp. i 



Exp. ii 



Exp. iii 



5439 

 5442 

 5440 



4492 

 4497 



4490 



947 

 945 

 950 



