Electric Lighting. 103 



of the instrument are so arranged that the amount of 

 current passing can be read off in amperes without the aid 

 of tables, and by the double arrangement of the wire-coil 

 the galvanometer is ten times more sensitive when joined up 

 in series than in parallel circuit, thus rendering the instru- 

 ment available for the measurements of small currents. 

 Perry and Ary ton's galvanometer has the advantage of being 

 portable, and is sufficiently accurate for ordinary testing or 

 ascertaining the amount of current passing through an 

 electric light circuit. It is particularly valuable in incan- 

 descent systems, because it becomes absolutely necessary to 

 keep the current constant and steady, and with such a 

 galvanometer in circuit, any alteration in the current strength 

 can at once be seen. For a temporary instalation of the 

 electric light, and where an engine not specialty adapted for 

 driving dynamo-machines is used, the galvanometer, if placed 

 near the engine-driver, serves the purpose of an auxiliary 

 governor. The engine-driver watches the galvanometer 

 needle, and having been directed to keep it at a certain 

 position, increases or diminishes the speed of the engine 

 according to any alteration in the galvanometer. This 

 method answered perfectly well recently at a lecture given 

 at home, the steady working of the Swan lamps shown tli ere 

 being entirely governed by the above arrangement. 



In my former paper I spoke with great distrust of Edison's 

 attempts at incandescent lamps. I felt justified in. the re- 

 marks then made, because really nothing but failures had 

 been recorded against him. Since then he has unquestionably 

 achieved a success, and produced an incandescent lamp, which 

 is stated to be quite equal to that of Swan. The specimens 

 of lamps before you are the " Original" and "New Swan," 

 the " British," and the " Maxim." These lamps are similar 

 in construction — a fine carbon loop in an exhausted and 

 hermetically sealed glass bulb. The Swan carbon is pre- 

 pared from ordinary crotchet cotton, treated with sulphuric 

 acid and carbonised. The Maxim is made out of cardboard, 

 Edison's is constructed from bamboo fibre, so that they may 

 all be classed as the same — all being converted into carbon. 

 But whilst their construction is apparently the same, their 

 behaviour and endurance differs slightly. Thus the old Swan 

 lamp should not be pressed beyond fifteen to twenty candle- 

 power, when it will last about 600 hours. The new form 

 has lasted over 1000 hours. The Edison lamps must not be 

 pressed beyond seven candle-power, whilst the Maxim ranges 



