356 REPORT— 1864. 



The measurements of lengths of wii-es tested M'ere made with a beam 

 compass. It was furnished with a vernier carrying a telescope. The in 

 strument was fixed horizontally before a window, the ends being clamped t' 

 shelves in the wall on either side of the window. 



The telescope pointed downwards, and the wires to be measured were laic 

 on a board fixed below the instrument. 



"With this apparatus measurements could be made with the greatest cer- 

 tainty to j'ijth of a miLlimetre, the telescope being sufficiently jjowerful t( 

 show much smaller lengths than this. 



We are indebted to Mr. B. Stewart for measuring the values of the divi- 

 sions of the instrument. 



Thermometers. — Two thermometers were employed. They were made bj 

 Messrs. Negretti and Zambra. One was divided to 4th of a degree Centi- 

 grade, the other to single degrees. The large thermometer was found to be 

 correct by the Kew standard. The zero-points of the thermometers Avere 

 carefuUy taken. 



Trou'/h. — The wires, the resistance of which were to be determined, were 

 jilaced in a glass tube immersed in a trough of water. 



The trough was 1-5 m. long by 0-15 m. square section. A stream of water 

 flowed through it, coming in by the tube V (fig. 1) and escaping by the waste- 

 pipe ^y. This arrangement was adopted because it was found that naphtha 

 or oil soon acted on the wires and altered their resistance, so that they could 

 not be immediately exposed to the action of a liquid. The details of the 

 arrangement wiU be understood by reference to fig. 4. 



The wire to be tested, a b, was soldered at its ends to copper bars as a c. 

 On to each of these bars was slipt a piece of glass tubing, as ef. These tubes 

 were fastened to the copper bars by india-rubber tiibing. The wire, with 

 its connexions, was then placed in the large glass tube A B. The piece of 

 tubing ef was then fastened to the bent tube CEDF by india-rubber tubing. 

 The ends of the terminals a c were beaten out flat and amalgamated. 

 The bent tubes were nearly filled with morcur^', and the terminal c was con- 

 nected with the mercury cups m\, in'^ of the instrument by copper rods amal- 

 gamated at each end. 



The resistances of the -wires were compared with those of coils of German 

 silver, well varnished, immersed in a cup of oU. The temperature of the oil 

 Avas determined by the small thermometer before described. 



Method of observing. — The wires at ere placed in the trough and the con- 

 nexions made. The water was then turned on and alloAved to flow for about 

 fifteen minutes. The large thermometer was placed in the trough, and the 

 temperature Avas read off by means of a lens placed so as to avoid all error 

 of parallax. The small galvanometer was then connected with the electric 

 balance, and the approximate reading foxmd. 



The large galvanometer Avas next connected, and the block handle pressed 

 down until any thermal current that existed had ceased to cause the needle 

 of the galvanometer to osciUate. The battery contact was then made for an 

 instant with the foot. The slight kick given by the spot of light at once 

 sliowed which Avay the block had to be moved, without its being necessary 

 to keep the battery on long enough to heat the conductors sensibly. 



The obsen'ing-room was kept at a very equable temperature by a screen 

 before the AvindoAV, also the wire of the balance was protected by a piece of 

 boarding from the heat radiating from the observer's body. 



After every observation the temperature of the coil and the water in the 

 trough was read off, and if any difference was observed between these read- 



