192 



ELECTRIC CLOCKS 



I \ 



thus again established, that tho coefficients of elasticity, as deduced from the- vibra- 

 tions, come out higher than those derived from tho elongation. 

 BIiATERITE. See ELASTIC BITTJMKN. 



EL ATERiuivi. A peculiar extract obtained from the juice of tho wild cucumber. 

 (Monnordica elaterium.) It owes its properties to a crystallisable principle called 

 Elaterin. 



EIiAlTIi. See OLEFIANT GAS. 



ELDER. (Bureau, Fr. ; Holwider, Ger.) Sambucus nigra. Pith-balls for elec- 

 trical purposes are manufactured from tho pith of the elder-tree, dried. The wood is 

 employed for inferior turnery-work, for weavers' shuttles, netting-pins, and shoe- 

 makers' pegs. Its elasticity and strength render it peculiarly fitted for these latter 

 purposes. The flowers of the elder are largely collected for distillation, yielding tho 

 well-known elder-flower water ; 9 pounds of flowers distilled with 4 gallons of water 

 give 3 gallons of the fragaut distillate ; they are also heated in lard for making 

 elder-ointment. The berries or fruit of the elder are employed for making one of 

 the truly British wines. 



ELECTIVE AFFINITY. (Wahlverwandtschqft, Ger.) See DECOMPOSITION ; 

 EQUIVALENTS. 



EIiECTRIC CZiOCKS. The application of electricity as a motive power to 



clocks, and as a moans of transmitting syn- 

 chronous signals or time, is naturally inti- 

 mately connected with the attempts (not yet 

 realised in an economic point) to apply it as 

 a motive power to machinery, and with its 

 application, so fully realised to telegraphy 

 proper (see article ELECTRO-TELEGRAPHY); 

 and it has grown up side by side with the 

 latter. Prof. Wheatstone's attention was 

 directed to it in the very early days of tele- 

 graphy. Without entering upon the history 

 of electric clocks, it will suffice to describe 

 two principles on which they have been con- 

 structed, and which are best known : Bain's 

 and Shepherd's. In the former, electricity 

 maintains the pendulum in motion, and the 

 pendulum drives the clock- train ; in the latter, 

 the motion of the pendulum is maintained 

 by electricity, but the clock-train is driven by 

 distinct currents, sent to it by means of 

 pendulum contacts. 



The bob of Bain's pendulum consists of a 

 coil of wire, wound on a bobbin with a hollow 

 centre. The axis of tho bobbin is horizontal. 

 Bar magnets, presenting similar poles, are 

 fixed on each side of the coil, in such a posi- 

 tion that, as the pendulum oscillates right and 

 left, the poles on either side may enter the 

 coil of wire. It is one of the laws of electric 

 currents, when circulating in a helix, or spiral, 

 or coil, or even in a single ring, that each 

 face of the coil presents the characters of a 

 magnetic polo ; of a south pole, if the current 

 circulates in tlio direction in which the hands 

 of a watch move, of a north pole, if it circu- 

 lates in the reverse direction. Things are so 

 arranged in Bain's pendulum that a battery 

 current is alternately circulating in and cut 

 off from the coil. When tho current is circu- 

 lating, the coil has the character of a magnet, 

 with a north end and a south end ; if the 

 permanent magnets present north poles, the 

 north end of the coil-bob will bo repelled from 

 one of tho magnets, while its south end will 

 z \ T* * ; c . be attracted by tho other magnet. This con- 



* * * stitutes tho impulse or maintaining power in 



one direction. Now the connections are such that, when tho arc of vibration is com- 

 plete and the pendulum ready for the return vibration, the pendulum rod pushes aside 



* 



