Experiments on the Faure Accumulator. 41 



-within it, and so the more curved front surface magnifies the 

 smaller image, which therefore appears unchanged. 



While on the subject of reflection in lenses, I think it worth 

 while to mention that convex lenses silvered at the back make 

 excellent and easily constructed concave mirrors. Since both 

 surfaces conduce to bring light to a focus, flatter curves may 

 be used than are necessary for a plain concave reflector of the 

 same focal length; also, since the two surfaces are not parallel, 

 false images are not produced ; so that the advantage of glass 

 silvered at the back remains, without the usual disadvantage. 

 A spectacle-lens of about five inches focal length, silvered at 

 the back and mounted, forms an eye-glass (I mean a glass for 

 examining the eye) which every one who works in metal should 

 possess. I have found by its means specks of metal, thrown 

 from the lathe, which were utterly invisible by other means, 

 but which were nevertheless exceedingly painful. 



III. Experiments on the Faure Accumulator. 

 By Professors W. E. Ayrton and John Perry*. 



HAVING made, at the request of the Faure Accumulator 

 Company, a series of experiments on some of their 

 cells, we have thought that a short account of some of the 

 results obtained may not be uninteresting to the members of 

 the Physical Society. 



The object of the experiments was to ascertain, first, the 

 efficiency of a cell — that is, the ratio of the energy given out 

 by it to the energy put into it; secondly, the storing-power of 

 a cell; and, lastly, whether or not there was a deterioration in 

 its working-powers. To measure the energy put into any 

 electric circuit, we have merely, of course, to take time- 

 readings of the current flowing through the circuit, as well as 

 the difference of potentials between its two extremities. The 

 current in amperes multiplied by the electromotive force in 

 volts and by 44-25, gives the number of foot-pounds per 

 minute that is being put into that part of the circuit as elec- 

 tric energy. For measuring the current we have used through- 

 out our ammeters (short for ampere-meters), and for measuring 

 electromotive force our voltmeters, the latter being employed 

 of course in a shunt circuit. 



Of the total electric energy put into the circuit, and which 

 is measured, in foot-pounds per minute, by 44*25 AV, a por- 

 tion will be employed simply in heating the circuit, and the 



* Communicated by the Physical Society, having been read at the 

 Meeting on February 25, 1882, 



