Resistance of Metals and Alloys. 277 



lens at the back of the substage, and an image of the wire 

 projected by the objective on to a sheet of fine hot-pressed 

 drawing-paper placed a yard away from the lens. The ob- 

 jective which formed this image always remained fixed rela- 

 tively to the screen, and the image was focnssed by moving 

 the substage which carried the wire. 



The magnification of the image was about 300 times linear, 

 so that the image of a wire about '003 inch in diameter could 

 be made to appear one inch in diameter. A 2-inch, 1-inch, 

 or J-inch objective could be used as desired. This optical 

 image or shadow could be made exceedingly sharp and well- 

 defined, and could be measured with a pair of dividers to 

 much less than one per cent. In order to infer the real dia- 

 meter of the wire from the diameter of the image, the follow- 

 ing method was employed. A series of five steel-wire gauges 

 was made formed of round parallel steel wire about 10 centi- 

 metres in length, rammed into wooden handles. These 

 handles were planed off on one side flat. These gauges varied 

 in diameter from about 3 millim. to '04 of a millimetre. The 

 gauges were sent to the Board of Trade Standardised Labora- 

 tory, and the diameters of the steel wires officially measured 

 with their Standard instruments along axes parallel to, the flat 

 side of the handle, and taken at the centre of the wire. This 

 being done, we had a series of standard gauges whose dia- 

 meter in one definite position was known to the one ten- 

 thousandth of an inch. These gauges were marked A, B, 

 C, D, E. 



Two more similar gauges were prepared, called F and G, of 

 smaller steel wire, and a number of observations of the dia- 

 meters of F and G relatively to E were made by means of a 

 micrometer-microscope belonging to the Physical Laboratory 

 of University College. Many observations were also made of 

 the relative diameters of the images of gauges E, F, and G 

 when projected by the projection microscope, the gauges being 

 in the same focal position relatively to the objective and screen. 

 These experiments, which were most carefully made, sufficed 

 to establish the diameters of the steel gauges F and G in the 

 centre and parallel to the flat side of their handles, in terms of 

 the Board of Trade measuring-instrument, and the final mean 

 result expressed in centimetres, is that the central diameters 

 of gauges F and G, parallel to the flat side of the handle, are 

 as follows : — 



Central diameter of gauge F = 0*03966 of a centimetre. 

 Central diameter of gauge G = 0*03959 of a centimetre, 



taken at about the normal temperature 15° C. 



