102 Electric Lighting. 



incandescent lamp, and consists of a slender pencil of carbon, 

 the point of which impinges on a copper button. The 

 success it lias met with is not due to the principle, which is 

 old, but to the thought and care bestowed in carrying out 

 mechanical details of construction in rendering the lamp 

 certain and automatic in its action. 



The use of powerful currents demands a special system of 

 testing. The ordinary methods adopted for battery-testing 

 cannot always be conveniently used, although of course they 

 are the most reliable ; but as all of them would involve the 

 use of shunts and a very delicate apparatus, special galvano- 

 meters or dynamometers have been introduced, which give 

 fairly approximate measurements. Siemens dynamometer 

 consists of a coil of wire through which the powerful current 

 is sent, and passes by means of mercurial contacts through a 

 rectangular frame of stout copper wire, suspended by a spiral 

 steel spring at right angles to the wire coil. The instrument 

 being set at zero, the current will deflect the wire-frame, the 

 opposing force being the torsion of the spiral spring. The 

 angle of torsion through which the spring has to be moved 

 in order to bring the wire frame to its zero point, can be 

 read off on the dial, and its value having been previously 

 determined by experiment and a table prepared, the amount 

 of current in amperes flowing through the instrument can at 

 once be ascertained. The accuracy of this instrument de- 

 pends entirely on the stability of the spiral spring; and in 

 order to render it as stable as possible, it is made of finely 

 tempered steel wire and then gilded. 



The absence of permanent magnets in this instrument 

 renders it of use for measuring alternate currents, for which 

 purpose another coil of thinner wire is provided. Another 

 form of galvanometer, lately introduced by Professors Perry 

 and Ayrton, is a modification of the Despres galvanometer, 

 in which a very small compound magnet needle is placed in 

 the field of a powerful permanent horse-shoe magnet. The 

 needle is also in the centre of a small wire coil, constructed 

 of a strand of ten wires, and so connected with a com- 

 mutator that the current can be either sent in parallel circuit 

 — that is, with the wires joined together at each end, and 

 representing a short and thick wire — or in series, which 

 would be the wire joined in continuous circuit, of one-tenth 

 the size, but ten times as long as in the former case. 



The action of this galvanometer renders the needle perfectly 

 dead beat, returning to zero quickly, whilst the proportions 



