310 REPORT— 1865. 



temper, they may be injured chemically in course of time by some action on 

 their surface ; it is just possible that the repeated passage of currents may 

 alter them in some way, although we have no reason as yet to expect such 

 an alteration. 



Mercury is free from all these objections. Its temper cannot vary, and as 

 it would be purified afresh on each occasion, it will be chemically uninjured. 



On the other hand, some fresh dangers may occur in its use. The tubes 

 themselves may alter in time, or the mercury may not always be absolutely 

 pure. Absolute security cannot be had, but the choice of a variety of 

 materials will probably prevent any serious alteration from occurring without 

 detection. 



The copies which have been issued are similar in form to the standard 

 coils, but the terminals are simple thick copper rods, intended to be dipped 

 in mercury cups. The security given by this mode of connexion is sufficient 

 for all ordinary purposes, and it was feared that the use of the double termi- 

 nals might not be everywhere understood. The platinum- silver alloy has 

 been used in all the copies. Wire made of this alloy is very strong and 

 ductile. It can, for instance, be drawn down to a diameter of 0-0002 inch. Its 

 resistance is not permanently altered even by a great change of temperature, 

 and even annealing hardly affects it. Moreover, the change in its resistance 

 due to a variation of 1° Centigrade is at ordinary temperature only 0-032 per 

 cent., being less than that of any other alloy tested. It is also a commercial 

 alloy, which has been long used by dentists ; and Dr. Matthiessen points out, as 

 a curious coincidence, that many commercial alloys coincide with proportions 

 indicating peculiar electrical properties. Vide Appendix A. 



The copies of the standard have been supplied for £2 : 10s. in boxes with 

 small mercury cups for the connexion, and with a printed direction for use 

 inside the box, stating the temperature at which that particular coil is equal 

 to 1 B.A. unit. 



A satisfactory proof of the accuracy with which these coils have been pre- 

 pared was given by four independent observations, by practical electricians 

 not belonging to the Committee, of the relative value of four distinct B.A. coils, 

 and four independent standards issued by Dr. Siemens. 



These four observations gave of 10456, 10455, 10456 and 10457 as the 

 measure of Siemens's standard, in terms of the B.A. units, proving the accuracy 

 both of Dr. Siemens's work and that of the Committee. 



Twenty coils were to be distributed gratis, and seventeen have actually 

 been given away to the following recipients : — ■ 



The Directors of Public Telegraphs in — 



France. Spain. Prussia. 



Austria. Italy. Sweden and Norway. 



Belgium. Portugal. Russia 



India. Victoria. 



Queensland. New South Wales. 



Also to Professor Kirchhoff, Dr. Joule, Professor Neumann, and Professor 

 Weber. 



Three remain for distribution. Sixteen have been sold. Dr. Faraday, on 

 behalf of the Royal Institution, was the first purchaser. 



In distributing the coils, it was thought best not to give them to institu- 

 tions, where they woidd probably have laid on a shelf useless and unknown, 

 but rather to distribute them widely, where they might become available to 

 practical electricians. 



