Comparisons of Mercury and Platinum Thermometers. 9 



The bridge-wire W W was made of platinum-silver, was 

 30 cm. long, and had a resistance of about 0'30 ohm. Con- 

 tact was made with the galvanometer through a similar wire 

 stretched parallel to the bridge-wire. The point of contact 



was determined by a very carefully constructed contact-key 

 carrying a vernier reading directly to -£■$ mm., so that yoo mm. 

 was easily estimated (corresponding to about 0*00001 ohm). 

 This ingenious contact-key, which was designed by Mr. Horace 

 Darwin, renders any injury to the bridge-wire almost im- 

 possible. The scale and bridge-wire are connected in such a 

 way that the readings are practically unaffected by very 

 considerable changes in temperature. The leads to the coil 

 of the platinum thermometer were connected to the terminals 

 P P and the compensating leads to the terminals C 0. The 

 galvanometer is shown at G, and the battery at B. 



When the intervals C to C and P to P were short-circuited 

 by strips of copper, and all plugs inserted, the bridge was 

 balanced when contact was made with the bridge-wire at 

 some point very near the centre, 0. The reading of the 

 bridge-wire when the bridge was balanced under these con- 

 ditions is called the zero correction to the bridge- wire. The 

 platinum thermometer was then inserted, and its resistance at 

 any temperature could be measured by suitable combinations 

 of coils and bridge-wire. The corrections to reduce the 



