46 Captain Abney on the Alkaline Development 



about 20° or 25°; in a tangent-galvanometer the most favour- 

 able position would be at 45°. Throughout these experiments 

 the effect always increased with the velocity of the contact- 

 breaker up to a certain point, about 100 per second, and then 

 declined. The general increase is in accordance with the ex- 

 planation here advanced, while the falling off might be owing 

 to an imperfect action of the make-and-break machine when a 

 certain velocity is reached. I am more inclined, however, to 

 attribute it to a want of theoretical softness in the iron, which 

 prevents it from taking the full magnetism when the alterna- 

 tion of currents is too rapid. In support of this opinion I 

 adduce one more experiment. Returning to the first arrange- 

 ment, in which the galvanometer was placed in the second 

 circuit, I arranged a third circuit in the neighbourhood of the 

 second or galvanometer circuit, whose ends could be joined or 

 kept apart *. In the second case, of course, no effect is produced 

 by the third circuit ; but what will be the result of completing 

 it? It is known that while the magnitude of each instantaneous 

 current in the galvanometer circuit is unaffected, the duration 

 of them is increased by the induction. This dilution, so to 

 speak, of the induced currents diminishes their heating-power, 

 which depends on the integral of the square of the current 

 while it lasts, and would, if the iron were perfectly soft, dimi- 

 nish the deflecting force on the galvanometer-needle ; but it 

 was found, on the contrary, considerably to increase it. In 

 fact, the induced currents are too condensed to produce their 

 full action, passing away before the needle is properly mag- 

 netized. 



It is too soon to say whether any use can be made of these 

 results ; but it is possible that such a soft-iron galvanometer 

 might be available for measuring the currents produced by 

 the new magneto-electric machines when the consecutive waves 

 are opposite. 



VII. On the Alkaline Development of the Photographic Image. 

 By Captain Abney, R.E., FM.S.-f 



SINCE alkaline solutions have been introduced for the 

 development of the photographic image, there has been 

 a certain amount of ambiguity regarding their action. It has 



* There were two similar coils, each containing two wires A 1? A 2 , B, , B 2 . 

 The battery-circuit included A 1 and the interrupter. The second circuit 

 consisted of the wires A 2 , B, and the galvanometer. B 2 gave the third 

 circuit, which was closed or open, according as the ends of B 2 were in 

 connexion or not.— Note added Dec. 1876. 



t Communicated by the Author. 



