152 



NATURE 



\_Dcc. 19, 1878 



the I2th inst., an additional sum of 250/. was voted for 

 their prosecution, and that they will be actively resumed 

 as soon as Dr. Greenfield has completed his arrangements. 

 The interval of inactivity has been used by the 

 committee for carrying out important improvements of 

 the premises in Wandsworth Road, so that Dr. Green- 

 field will enter on his new duties with many advantages 

 in his favour— an excellent laboratory, sufficient re- 

 sources, fruitful work already in progress, and a com- 

 mittee including such men as Busk, Gull, Paget, Ouain, 

 Sharpey, and Simon to back him. We feel confident 

 that the wisdom of the appointment will be justified by 

 the result, and that the new chapter in the history of the 

 Brown Institution which will begin with the year 1879, 

 will be a successful one. 



ON SOME IMPROVED METHODS OF PRO- 

 DUCING AND REGULATING ELECTRIC 

 LIGHT'' 



I 



N a former communication to the Society I directed 

 attention to the fact that when the electric hght is 

 produced from the ends of two carbon pencils placed 

 parallel to each other, if the strength of the electric cur- 

 rent, the thickness of the carbons, and the distance 

 between them are rightly proportioned, the carbons will 

 burn steadily downwards until they are wholly consumed, 

 without any insulating material between them. To ini- 

 tiate the light by this method, it is necessary to complete 



1 



the electric circuit between the carbons by means of some 

 conducting substance, which volatilises on the passage of 

 the current, and establishes the electric arc between the 

 points. 



When a number of such lights are produced simulta- 

 neously from the same source of electricity, any interrup- 

 tion in the continuity of the current extinguishes all the 

 lights in the same circuit, and each pair of carbons re- 

 quires to be reprimed before the lights can again be 

 estabhshed. This defect, as will be obvious, would cause 



* Supplement to Paper read at the Manchester Literary and Philosophical 

 Society, November 26 (see Nature, vol. xix. p. 78). Communicated by the 

 Author. 



great inconvenience when the lights are not easjly acccs-* 

 sible, or are at considerable distances apart/ • r. - , 



In the course of my experiments it was observed that 

 when the electric circuit was completed at the bottom of 

 a pair of carbons close to the holders, the arc immediately 

 ascended to the points, where it remained so long as the 

 current was transmitted. My first impression of this pecu- 

 liar action of the arc was, that it was due to the ascending 

 current of hot air by which it was surrounded. This, 

 however, was found not to be the cause, as the arc 

 travelled towards the points in whatever position the 

 carbons were placed, whether horizontally or vertically xty 

 an inverted position. Moreover, when a pair of carbons 

 were held in the middle by the holders, the arc travelled 

 upwards or downwards towards the points, according as 

 the circuit was established above or below the holders. 

 The action was, in fact, recognised to be the same as that 

 which determines the propagation of an electric current 

 through two rectilinear and parallel conductors submerged 

 in contact with the terrestrial bed, which was described 

 by me in the Philosophical Magazine, August, 1 868. 



In all the arrangements in general use for regulating 

 the electric light, the carbon pencils are placed in the 

 same straight line, and end to end. When the light is 

 required, the ends are brought into momentaiy contact, 

 and are then separated a short distance to enable the arc 

 to form between them. The peculiar behaviour of the 

 electric arc when the carbons are placed parallel to each 

 other, suggested to me the means of lighting the carbons 

 automatically, notwithstanding the fact that they could 

 only be made to approach each other by a motion laterally, 

 and to come into contact at their adjacent sides. Ta 

 accomplish this object, one of the carbon holders is 

 articulated or hinged to a small base plate of cast iron, 

 which is so constructed as to become an electro-magnet 

 when coiled with a few turns of insulated wire. The 

 carbon holder is made in the form of a right-angled lever, 

 to the short horizontal limb of which is fixed an armature 

 placed over the poles of the electro-magnet. W^hen the 

 movable and fixed carbon holders are brought into juxta- 

 position, and the carbons inserted in them, the upper 

 parts of the two carbons are always in contact when no- 

 current is transmitted through them, as shown by the 

 dotted lines in the engraving. 



The contact between the carbons is maintained by 

 means of an antagonistic spring inserted in a recess in 

 one of the poles of the electro-magnet, and reacting on 

 the under side of the armature. One extremity of the 

 coil of the electro-magnet is in metallic connection with 

 the base of the carbon holder, while the other extremity 

 of the coil is in connection with the terminal screw at the 

 base of the instrument from which it is insulated. The 

 coils of the electro-magnet are thus placed in the same 

 circuit as the carbon pencils. 



When the alternating current from an electro-magnetic 

 induction machine is transmitted through the carbons, 

 the electro-magnet attracts the armature and separates 

 the upper ends of the carbons, which brings them into 

 their normal position, and the light is immediately pro- 

 duced. When the circuit is interrupted, the armature is 

 released ; the upper ends of the carbons come into con- 

 tact, and the light is produced as before. When several 

 pairs of carbons are placed in the same circuit, they are, 

 by this arrangement, lighted simultaneously. 



H. Wilde 



INFLUENCE OF THE STRAITS OF DOVER 

 ON THE TIDES OF THE BRITISH CHAN- 

 NEL AND THE NORTH SEA ' 

 THE conclusions are : — 

 I. The rise and fall of the water-surface and the 

 tidal streams throughout the North Sea north of the 



' Abstract of a paper read at the Dublin meeting of the British Assccia- 

 tion. 



