308 report — 1865. 



Postscript. — Since the above Report was drawn up, an outline Map of the 

 Moon, of 100 inches in diameter, has been commenced, and is now in pro- 

 gress. Tbe positions of 105 points of the first order, as given in Beer and 

 Madler's ' Der Mond,' pp. 77, 78, form the basis of measurement for all the 

 features discernible upon the photograph taken by Warren De la Rue, Esq., 

 on October 4th, 1865, a short time before contact with the Earth's shadow, 

 and when the apparent disk was not far from a state of mean libration. In 

 this Map every important feature of the Full Moon will be laid down from 

 measurements on the photograph referred to the equator and central meri- 

 dian. 



Report of the Committee on Standards of Electrical Resistance, 



The Committee consists of Professor Williamson, Professor Wheatstone, Pro- 

 fessor W. Thomson, Professor Miller, Dr. A. Matthiessen, Mr. Fleeming 

 Jenkin, Sir Charles Bright, Professor Maxwell, Mr. C. W. Siemens, 

 Mr. Balfour Stewart, Dr. Joule, and Mr. C. F. Varley. 



The Committee has the pleasure of reporting that the object for which they 

 were first appointed has now been accomplished. 



The unit of electrical resistance has been chosen and determined by fresh 

 experiments; the standards have been prepared, and copies of these standards 

 have been made with the same care as was employed in adjusting the stan- 

 dards themselves; seventeen of these copies L have been given away, and 

 sixteen have been sold. 



The chief work of the Committee this year has been done by Dr. A. Mat- 

 thiessen. Last year's Report announced the completion of the experiments 

 determining the resistance in absolute measure of a certain coil of German- 

 silver wire. Taking this coil as the basis, Dr. Matthiessen, assisted by Mr. C. 

 Hockin, prepared ten standards, each expressing the British Association unit 

 of electrical resistance ; two of these standards are coils of platinum wire, 

 two are coils of wire drawn from a gold-silver alloy, two are coils of wire 

 drawn from a platinum-iridium alloy, and the remaining two are tubes of 

 mercury. 



The wires employed in the coils are from 0'5 millini. to 0-8 millim. dia- 

 meter, and range from one to two metres in length. They are insidated with 

 white silk, and are wound round a long hollow bobbin of brass. The wires 

 are imbedded in solid paraffin, and enclosed in a thin brass case, which allows 

 the coils to be plunged in a bath of water by which their temperature may 

 be conveniently regulated and observed. Two short copper terminals project 

 from the case and are forked at their ends, so that they may be connected 

 with the Wheatstonc's balance in the manner recommended by Professor W. 

 Thomson, avoiding the error due to the possible resistance of connexions. 

 The mercury standards consist of two glass tubes about three-quarters of a 

 metre in length. 



These ten standards are equal to one another and to the British Association 

 unit, at some temperature stated on the coil or tube, and lying between 14°-5 

 andl6°-5C. 



None of them, when correct, differ more than 0-03 per cent, from their 

 value at 15°-5 C. 



In the choice of the material, of which the standards are constructed, the 



