SO.Mi: MKIiCUKY STANDARDS OF UESISTANCK, ETC. 103 



ensure cleanliness of the interior. In the spare aperture a sensitive thermometer 

 was placed, but the arrangements at the neck were such that a platinum thimble 

 could be inserted if necessary. Shellac varnish was applied to all junctions 

 to ensure good insulation of the mercury in the tube and vessels. Long copper leads 

 were made so as to connect any two of the thimbles to two mercury cups of the 

 Carey Foster bridge, one end of each lead being shaped so as to fit any one of the 

 thimbles. Two compensating leads, also of copper, connected the two opposite 

 mercury cups to the standard coil employed, the difference in resistance of the leads 

 on opposite sides of the bridge amounting to about '00006 ohm. 



Each platinum thimble had its entire surface amalgamated in the following manner. 

 The platinum was first scoured with acids, and afterwards heated to redness in a 

 Bunsen flame. Platinisation was then effected by the passage of a current of 

 electricity through a solution of platinum chloride containing a small amount of lead 

 acetate. Afterwards the platinum thimbles were heated to redness, and plunged 

 whilst hot into mercury. Amalgamation immediately resulted, and thoroughly good 

 contact between the copper and the mercury was thus secured. When the platinum 

 thimbles were in position in the glass vessels, the distance from one end* of the 

 standard tube to the nearest platinum thimble was l centims. 



The tube and vessels being filled with pure mercury, and the necessary accessories 

 fitted, the whole was placed in the ice box, and the first observation of resistance 

 made 12 hours afterwards. The ice was packed well alxmt the sheaths, the upper 

 surface being quite 8 centims. above the necks of the vessels. The copper leads were 

 introduced into L, and L.,, cotton wool forming a collar about the rods at the point 

 of emergence from the sheaths. 



The necessary interval haying elapsed, the temperature of the mercury in the vessel 

 containing the thermometer was read, and the leads connected to the mercury cups of 

 the bridge, the standard coil and its leads being already in position. An observation 

 was now taken of the difference 



Resistance of mercury standard plus leads Resistance of standard coils plus leads. 



Immediately this measurement was completed, the lead immersed in Lj was rapidly 

 removed to Lg, the leads of the coil being at the same time disposed so as to eliminate 

 the resistance of the coil from the measurements. The difference 



Resistance of leads of mercury standard Resistance of leads of standard coil 



was then ol>served on the bridge. In this way, assuming all of the platinum thimbles to 

 be similar, the resistance of the leads and thimlilrs was eliminated. A reversal to 

 the former conditions was now effected, and the previous observation confirmed. 



The recorded temperature of the mercury in the end vessels was always considerably 

 above C. ; it averaged I '3 (J. The temperature of part of the mercury in the tube 



