THE DEVELOPMENT OF ELECTRICAL SCIENCE. 227 



of the discovery, by May aucl Smith, of the effect of light on the resist- 

 ance of selenium. This effect has since been found in the case of a 

 large number of other substances, but it is still an interesting field for 

 research. A number of experiments on this subject have been associ- 

 ated with attempts to make things visible at a distance. No doubt it 

 will ultimately be possible not only to talk to a distant party, but also 

 to see the party talked to, and thus, as it were, look the party with 

 whom you are conversing in the eye. 



The subject of telegraphy is closely associated with the present excel- 

 lent system of electrical measurements aud with the invention of many 

 of our most delicate measuring instruments. As the applications of 

 electricity increased there gradually grew up a new branch of engineer- 

 ing, a branch, however, in which the foot rule, pound weight, chro- 

 nometer, and thermometer were not sufficient. Other standards of meas- 

 urement were required, in order that quantities could be gauged and 

 consistent work done. The way to connect the measurements of the 

 new quantities with the units already in use in dynamics had been 

 pointed out by Gauss and others, and at the suggestion of Thomson 

 the British Association appointed a committee in 1861 to determine the 

 best standard of electrical resistance. This led to an unexpected 

 amount of work, not only on a standard of resistance, but also on the 

 general subject of electrical measurement. The committee regretted, 

 at the end of the first year, that it could not give a final report, but 

 hoped that the inherent difficulty aud importance of the subject would 

 sufficiently account for the delay. It can hardly be said that the final 

 report has yet been forthcoming, as a committee with some of the origi- 

 nal members in it still exists and reports regularly every year on valu- 

 able work done by it. The committee worked energetically for a number 

 of years, not only on the standard of resistance, but on those of cur- 

 rent, electro- motive force and capacity. It incidentally supplied a great 

 deal of quantitative data on a number of subjects, and particularly as 

 to the permanence of alloys, the variation of their resistance with tem- 

 perature as depending on their composition, and so forth. In looking 

 over the results of the early work of the British Association committee 

 one is apt to indulge in adverse criticism. It is hard for many of the 

 younger workers to appreciate the difficulties which are met in a first 

 attempt. It would be equally just to congratulate ourselves that we 

 have better marksmen to-day than there were fifty years ago, with- 

 out making allowance for the modern rifle. 



The first absolute determination of resistance was probably that 

 made by Kirchhoff about fifty years ago. Weber published his method in 

 1852, and then came the British Association determination by Maxwell, 

 Stewart, and Jenkin in 1863. Neither of these was very exact, but they 

 paved the way for the splendid exhibitions of experimental skill which 

 followed. Among those to whom we are most indebted for this later 

 work may be mentioned Kohlrausch, Rayleigh, Glazebrook, Rowland, 



