264 



THE POPULAR EDUCATOR. 



Fig. 34 



VOLTAIC ELECTRICITY. VI. 



DIFFERENT FORMS OF ELECTRIC LAMP : DTJBOSCQ'S, SERRIN's, 

 ETC. PROPERTIES OF ELECTRIC LIGHT. 



OWING to the intense power of the electric light, and its conse- 

 quent usefulness for the illumination of lighthouses and public 

 places, and also for exhibiting small objects by means of the 

 lantern, very many attempts have been made to devise a perfect 

 lamp, so as to avoid the flickering and variations in the light 

 which are seen when the apparatus first described is used, it 

 being evident that these form a serious hindrance to its employ- 

 ment. At present we shall 

 notice only some of the earlier 

 kinds, while in a later lesson 

 on electro - motive machines, 

 we shall mention some of the 

 most important of the new 

 inventions which follow one 

 another with great rapidity. 



In many forms the carbon 

 poles are fixed to brass racks, 

 the teeth of which work in 

 wheels fixed on the same axle, 

 but of different sizes, so that 

 the upper or positive pole may 

 move at twice the speed of the 

 lower. In some instruments 

 the weight of the upper pole, 

 and the rod which supports it, 

 is sufficient to raise the lower 

 pole ; in others both are 

 driven by clockwork. In 

 either case their motion is 

 arrested by means of an 

 electro-magnet as soon as the 

 current is established between 

 the charcoal points. 



A lamp which is well 

 known, and which was for a 

 long time the only one by 

 which a steady light could 

 be obtained, was devised by 

 M. Duboscq, and is called by 

 his name. It is represented 

 in Fig. 33. The racks a 

 and b are attached to the 

 upper and lower poles respec- 

 tively ; the latter works in 

 the teeth of the small wheel 

 E, while the other works in 

 the wheel F, which, has just 

 twice the diameter. Theso 

 wheels are on the same axle, 

 and press against one another 

 with sufficient force to cause 

 them to move together. Fre- 

 quently, however, it is required 

 to move only one pole so as 

 to adjust the height of tho 

 light, and this may be accom- 

 plished by holding the handle 

 p while that at the other side 



if the instrument is turned, the wheel E being attached to r>, 

 and the wheel F to the other handle. 



There is also a series of small wheels driven by the central 

 drum ; these give motion to a small fan, G, which regulates the 

 speed of the clockwork. A wheel placed horizontally under 

 this fan serves to stop the motion of the wheels, and there- 

 fore that of the points, as soon as the current begins to pass. 

 Th.8 is accomplished by means of the iron ring H and the rod 

 K, in the following manner : The lower part of the instru- 

 ment consists of a large bobbin, L, round which the current 

 passes. Inside this is a bar of soft iron, which is made into a 

 temporary magnet by the passage of the electricity round L. 

 The ring H is thereupon attracted and drawn down, and thus 

 by means of a lever raises the rod K, till a stud fastened to it 

 catches in the teeth of the fan- wheel and stops it. 



We will now trace the course of the current through the 



instrument. The positive wire is connected with the tube B 

 by means of the binding-screw A. A ring of ivory or ebonite 

 insulates this tube from the top of the lamp, and thus prevents 

 the electric current from finding its way through the clockwork 

 direct to the binding-screw M. The position of the upper char- 

 coal can easily be adjusted by means of the ball-and-socket 

 joint at c, and the rod which slides through it and carries the 

 carbon. Down this rod the electricity travels, and as the poles 

 are kept in contact, it at once passes to the lower one. Thence 

 it goes down a rod at the side to the bobbin L, and round this 

 to the binding-screw M. Insulating discs are employed to 



keep the current from passing 

 down the brass rack connected 

 with the lower pole. 



Now when the poles are 

 separated, the battery wires 

 may be connected ; if this be 

 done without separating them, 

 the screws will get injured by 

 the heat. The poles then 

 come together slowly, the rate 

 of motion being regulated by 

 the fan. As soon as they 

 touch, the ring H is attracted 

 by the magnet, K is raised till 

 the stud catches in the fan- 

 wheel, and thus all further 

 motion of the poles is pre- 

 vented. They would, however, 

 remain in contact, so that a 

 very faint light would be seen 

 until they had burnt away a 

 little ; an arrangement is ac- 

 cordingly made by which the 

 ring H, as it descends, draws 

 down the rack 6, and thus 

 lowers the pole enough to 

 display the light in its brilli- 

 ancy. The amount of this 

 motion has to be regulated so 

 as to correspond with the 

 power of the battery. The 

 light now remains steady for 

 a short time, but soon, as the 

 poles burn away, the distance 

 between them increases, and 

 the current becomes weaker - f 

 the magnet then loses a part 

 of its power ; the ring H is 

 raised by means of a spring 

 under it ; and the clockwork 

 being thus released, the poles 

 again approach till the current 

 is strong enough to overcome 

 the elasticity of the spring, 

 when the wheels are stopped 

 as before. 



When the apparatus is 

 properly adjusted to the 

 strength of the batteries, the 

 light from it does not vary 

 very greatly. The adjust- 

 ments, however, require some care ; and if the light be acci- 

 dentally extinguished, as it may easily be by a puff of air, 

 the poles only move slowly, and therefore an appreciable timo 

 elapses before it re-lights. A further drawback to the employ- 

 ment of this apparatus is its expense. 



Another form of lamp has been devised by Serrin, the main 

 advantage of which is that it can be lighted from a distance 

 by merely causing the current to pass through it. The weight 

 of the upper pole is in this the moving force, and thus the 

 poles always remain in contact when the current is not passing. 

 They are, however, so arranged that as soon as the points meet 

 the clockwork is locked, whereas in many lamps the points 

 are broken by being pressed upon each other. 



A very simple form of apparatus, which has been found to 

 answer well for a small battery, was brought out by Mr. Brown- 

 ing (Fig. 34). There is no clockwork at all in it, and therefore no 



