1875.] President's Address. 39 



traffic on this line has of late been so great that the question of putting up 

 a second wire had to be considered. 



The cost of this second line would have been about Rs. 4,00,000, but 

 this expenditure will be avoided by the introduction of the Duplex arrange- 

 ment, which will give equal capacity for traffic at a cost of about Rs. 4,000. 



In addition to this, there are many technical advantages connected with 

 the Duplex system of which it is- necessary to mention the entire elimination 

 of contacts and of the effects of Voltaic induction which is so much felt on 

 long direct-worked circuits. 



Action of Light on the Electrical Resistance of Selenium. — This is a 

 most important discovery. It has been ascertained that the electrical re- 

 sistance of crystalline selenium decreases considerably with the intensity of 

 light shining on the selenium. 



With a view to investigation on this head I have had an arrangement 

 made for perfectly insulating a bar of selenium in a box, which can be 

 opened and closed at pleasure from any distance of the observer, and Mr. 

 Schwendler has already made some qualitative experiments with the appara- 

 tus verifying Lieut. Sale's and Earl Ross's experiments, i. e., that the effect 

 is due to light and not to conducted or radiated heat. 



He found that the decrease of resistance of the pieces of selenium 

 experimented with, is unmistakable, since it represents a variation up to 67 

 per cent, between darkness and the light of an ordinary kerosine lamp 

 shining on it at a distance of about 13 inches. 



It is our desire to find by careful experiments, to which the very per- 

 fect electrical apparatus at the Government Telegraph Store-yard is so 

 eminently applicable, the law which connects the variations of resistance 

 with the intensity of the light. 



Mr. Schwendler expresses a belief that other similar substances, as for 

 instance, sulphur in its two different states, would shew a similar effect, only, 

 that it is more difficult to shew it, on account of the very small absolute 

 conductivity sulphur possesses. 



It is scarcely necessary for me to call your attention to the great future 

 usefulness of this discovery (which is quite independent of its most interest- 

 ing nature in revealing another co-relation of forces) I mean for the con- 

 struction of a rational Photometer for which such a great want is felt and 

 which does not at present exist. 



In fact with this practical object in view, I secured the bar of selenium 

 and constructed the apparatus mentioned. 



Unit of Electrical Resistance. — Professor Kohlrausch has shewn that 

 the British Association unit contains a probable error of about 2 per cent. 

 and therefore, as the Siemens or mercury unit approaches the multiple 

 absolute or Weber's unit, of which the British Association unit is to be a 



