468 REPORT— 1880. 



of force ' — naturally arranged — tlie great superiority of the circular magnet, where 

 an armature is to be employed. 



Since that time some thousands of that form of magnet have been made for 

 medical, mining, and other purposes. 



Some months ago, when in conversation with M. Breguet of Paris, I showed 

 him these same diagrams, and he was very much impressed with their importance. 

 lie has since then constructed a machine, using the Gramme armature ; and with a 

 smaller quantity of steel in the magnets he has made a far more powerful machine 

 than hitherto constructed with either the Jamin or the ordinary horse-shoe form. 

 It is also more symmetrical in appearance and occupies less space. 



With this machine I can heat to incandescence 19 inches of platinum wire 

 by four turns of the handle ; while to heat 14 inches of the same sized wire by a 

 machine having a Jamin magnet took ten turns of the handle. 



5. An Account of some ^Experiments in Photo-electricity. 

 By G. M. MiNCHiN, M.A. 



The two objects aimed at primarily in photo-electricity are — 



(rt) the production, at a distance, of effects due, in the first instance, to the 



photographic action of light ; 

 (b) the continuous daily registration of the intensity of sunlight of any 

 selected wave-length. 



The first of these is the problem of constructing what the author has called 

 the I'eleiyhotograjih, and some of the fundamental conditions of success have been 

 attained. 



The second problem will, in all probabiUty, soon attain a satisfactory solution, 

 much progress having been already made towards it. 



The author investigated, in the first instance, the photo-electric currents pro- 

 duced by the action of light on silver plates, coated with the ordinary emulsions 

 of silver salts in use among photographers — viz., the chloride, bromide, and iodide 

 of silver. 



In a cell containing tap water (or slightly acidulated water, or distilled water 

 with a few grains of common salt), if a chloride plate is immersed in presence of 

 an uncoated plate, the current runs from the latter to the former in the cell. 



The same is the direction of the current when the chloride is replaced by a 

 bromide plate. 



But if the sensitised plate is an iodide plate (the conducting liquid being dis- 

 tilled water with a few grains of iodide of potassium), the direction of the current 

 is reversed. 



In carrying out an idea about phosphorescence as a photo-electric source, it 

 appeared to be of importance to study sulphide of silver. If any emulsion of this 

 salt is made with collodion, and a silver-plate sensitised with it be immersed, as 

 above, in a glass cell, the direction of the cm-rent given by magnesium light (or 

 simlight), agrees with that of the iodide plate ; and by passing the light incident 

 on the plate through coloured glasses, it will be found that the red and the blue 

 rays give strong results in the same direction, while the green light gives a com- 

 paratively trifling action. For this salt there is therefore a point of minimum 

 sensibility in the middle of the spectrum. A silver plate coated with nitrate of 

 silver (shaken up in a test-tube with thin photographic gelatine), gave with blue 

 rays a strong current in the iodide and sulphide direction ; and with red rays a 

 very small result in the opposite direction, though whether this latter result is due 

 to the action of the red rays on the emulsion or on the plate itself is not 

 certain. 



It was found in several of these experiments that the observation of Grove to 

 the effect that light sets up a current in the direction of some previously existing 

 current, being incapable of setting up one of its own, was not confirmed. The 

 experiments of Grove which gave rise to this statement are referred to. From 



