500 



Sir James N. Douglass. 



[June 10, 



J/8°, then the deflection from the new zero will be less still ; so that 

 there is no advantage in the use of the new zero unless the deflections 



exceed 



If the angle (3° be made greater than 60° then the possible angular 

 movement of the needle becomes still further increased ; but inasmuch 

 as any increase in the length of the tangent scale brings the divisions 

 at the ends of the scale proportionally closer together, a.nd makes 

 them more difficult to read from, there would be no practical 

 advantage in making the angle larger. 



A marked advantage under certain conditions is found when the 

 new zero has such a value that the deflection from a given current 

 causes the needle to move up to the ordinary zero, that is to say, to 

 the position where the needle becomes parallel to the coil ; in this 

 case the instrument becomes highly sensitive, and any increase in the 

 strength of the current produces a very considerable change in the 

 deflection. 



VIII. « On Fluted Craterless Carbons for Arc Lighting." By 

 Sir James N. Douglass. Communicated by Sir William 

 Thomson, F.R.S. Received June 4, 1886. 



[Plate 6.] 



On the 8th December, 1858, at the South Foreland High Light- 

 house, and with the direct current magneto machines of Holmes, the 

 first important application of the electric arc light, as a rival to oil 

 and gas for coast lighting, was carried out by the Trinity House, 

 under the advice of Faraday. The carbons then used, and for several 

 years afterwards, were sawn from the residuum carbon of gas retorts ; 

 they were square in section, 6 J X 6^ mm., and the mean intensity of 

 the arc, measured in the horizonal plane, was 670 candle units, being* 

 17 candle units nearly per square millimetre of cross sectional area of 

 the carbon. The crater formed at the point of the upper carbon of 

 the " Holmes " lamp was so small that no appreciable loss of light 

 was found to occur, and the arc proved to be very perfect in affording 

 an exceptionally large vertical angle of radiant light for application 

 with the optical apparatus as shown, one-third full size, in the sketch 

 (Plate 6). 



The most reliable and efficient machine that has yet been tried for 

 lighthouse purposes is the large size alternate current magneto 

 machine of De Meritens. The average results with these machines 

 are as follows, viz. : — 



