BiCKERTON. — On a Modification of the Electric Lamp. 403 



The relays used in either of the foregoing systems are Siemens' polarized 

 relays, and all the values given are in Siemens' units. 



In the plan showing the system on which duplex is worked on the Ko. 3 

 wire in the Cook Strait cable, for convenience sake the local circuits are 

 omitted. 



Art. LXV. — On a Modification of the Electric Lamp for projecting the 

 Spectra of different Metals on the Screen. By A. W. Bickerton, 

 F.C.S., Professor of Chemistry in Canterbury College. 



lEead before the Philosophical Institute of Canterbury, 1st October, 1874. ] 



It is a matter of some difficulty, when using an ordinary electric lamp, to 

 project the spectra of several metals on the screen in quick succession. A 

 special piece of apparatus for this purpose is described by Mr. Lockyer in his 

 book, in which a circle of ordinary gas carbon cups are used. I have made 

 the following modification of the ordinary lamp, and have found the result 

 very successful. 



The lower carbon is taken out, and into the holder is placed a stout 

 cylinder of brass w^ith a pin turned on the top j on this pin is a plate of gas 

 carbon three inches in diameter and half-an-inch thick, with a hole in the 

 centre through which passes the brass pin. The carbon disc is thus free to 

 revolve round the top of the pin j near the edge is a circle of small hollows into 

 which the pieces of the metals are placed. The upper carbon would now come 

 in contact with the top of the brass pin ; in order to bring the upper carbon 

 above the hollows containing the metals, the top arm is provided with a slot 

 to slide out. With this arrangement the spectra of a number of metals may 

 be projected on the screen in a few minutes with the greatest ease, the metals 

 burning with an amount of steadiness I have not seen before. A piece of 

 copper about the size of half a pea was placed in one of the hollows and 

 completely burnt away without any attention. 



The lamp I have is one of Browning's, and it acts most satisfactorily, but 

 the positive carbon is placed at the top. I find that in order to burn the 

 metals with a steady light the positive must be at the bottom ; when, it is at 

 the top it invariably begins to vibrate after a little time, and sometimes throws 

 the metal out of the cup. I have not yet noticed if this is the case when 

 using the ordinary carbon cups. The steadiness of the arc is probably due to 

 the fact that the large mass of the carbon plate prevents it from becoming 

 much heated, so that the convection currents, which I believe to be the cause 

 of the arc (see my paper, Phil. Mag., Dec, 1873), have naturally a greater 

 tendency to spring from the heated metal than the cold carbon. So small is 

 the heat developed in the carbon that it remains quite black, and may be 

 safely turned round with the hand. 



