312 repokt — 1865. 



I. It alters less in electrical resistance with changes of temperature than 

 any other known alloy. 



The importance of this point needs hardly to be enforced on any one who 

 has used resistance-coils. 



The increment in the resistance of the alloy due to a change of tempera- 

 ture from 0° to 100° C. is only 3-2 per cent. 



II. The conducting-power of the alloy is very low, and is about one-half 

 that of German silver. 



III. The conducting-power of the alloy is not altered by baking, that is 

 by exposing it to a temperature of about 100° C. for several days. 



This is a property of great importance, for it has been observed that those 

 conductors which do not alter by baking, do not alter by age either. The 

 experiments by which this has been established have been published in for- 

 mer Reports. 



IV. The conducting-power of a wire of the alloy is little altered by 

 annealing. 



Further, the alloy does not oxidize by exposure to the air ; it does not readily 

 alloy with mercury ; it makes a sufficiently pliable wire, and can be drawn to 

 a very great degree of fineness. Dentists have made considerable rise of it 

 in consequence of its good chemical and mechanical properties*. Of this 

 alloy, twenty unit-coils have been made and sent to several leading electri- 

 cians at home and abroad. The form of bobbin adopted for putting up the 

 wire, and shown in Plate X. fig. 1, has been found very convenient, as it can 

 be immersed in water during an observation. The wire is twice coated with 

 silk, and protected by being imbedded in solid paraffin. 



Besides the coils already mentioned, ten unit-coils have been made, which 

 will be deposited at the Kew Observatory. 



Any one possessing a copy of the B.A. unit may have it compared at any 

 future time against one of these coils for a small payment. 



Of the coils to be sent to Kew, two are of the platinum-silver alloy, 

 two of the gold-silver alloy, two of a platinum-indium alloy, and two of 

 commercially pure platinum. Two mercury units have also been prepared. 



With so many coils for reference, made of such different metals, it appears 

 quite improbable that the unit now proposed should be lost. 



Along Avith the above-mentioned coils will be preserved the standard coil 

 used in the experiments first referred to, the coil used in the similar experi- 

 ments made by your Committee in 1863, and several copies of these coils. 



Of the coil called "June 4th" in the Report of your Committee for 1863, two 

 German-silver copies have been made. Of the other coil used in 1864, two 

 German-silver, two gold-silver, and one platinum-silver copy have been made. 



These coils have twice been recompared together at intervals of three 

 months, and will be again compared, and if they are still found not to have 

 altered, will be deposited at the Kew Observatory for reference, their values 

 being engraved on them. 



The method adopted to obtain the unit from the standard which had at a 

 certain temperature a resistance of 4-6677 B.A. units was this : — 



Coils were made with the following approximate resistances, viz. : 



Two coils nearly equal to one-half a unit, called ^(a) and g(b). 

 „ „ one unit „ 1 (a) „ 1 (b). 



One coil ,, two units „ 2 



„ ,, two and a half „ 2-i- 



* Messrs. Johnson and Matthey inform us that this alloy has heen in use for nearly 

 twenty years. 



