VOLTAIC ELECTRICITY. 



difficult; in making the adjustments. The lower pole A S fixed 

 t<. iii.- board, a certain amount of rertioal odjnatmei.' 

 ullowvd by means of sliding tube*. A pillar, K. curved at the 

 top, nerves to carry tho upper pule, which nlidea by its own 

 u.-i-lit through a abort tube 11. On tho upright ia an elf ' 

 magnet c, with a small keeper, i>, a little way abore it. Thin 

 >T ia fixed to one end of a lover, at the other end of whi'-h 

 ia a pin \\ iii.-li proves, through a slot in the tube u, agaiiut tho 

 .!. Aa soon aa tho current ia established, 



the keeper ia drawn down, and the pin then hold* the pole in it* 

 place. When, however, tho carbons have waatod so aa to 

 weaken the current, tho keeper ia loosened, and allowa the pole 

 to slip a little lower. A reflector ia shown attached to thin 

 lamp BO aa to concentrate the light. 



In thin apparatus there ia no arrangement to aeparate tho 

 polos, but as it is only intended for use with small battery 

 power, they do not need any great amount of separation, and 

 noon burn away sufficiently to give this. 



Another kin. I of lamp, invented some few years apo, may 



bo described 



here, as it 



:l ilif- 



: way 



from those 



ulrcii'ly incii- 



Uonedinthis 



lesson, iiinl 

 at tho time 

 of its in- 

 vention it 

 was a more 

 useful form, 

 besides bo- 

 ing much 

 cheaper 

 than most of 

 the others. 

 In this all 

 clockwork is 

 entirely dis- 

 pensed with, 

 the regula- 

 ting power 

 being deri- 

 ved from tho 

 pressure of 

 the air con- 

 fined in a 

 large india- 

 rubber Imll. 

 The weight 



and the cord being thus Blackened, the lower pole falls by 

 its own weight Aa, however, this post ia supported by two 

 cords, it ia clear that iU motion will only be half as rapid ae 

 that of the npper pole. When the polea are aefurated to a 

 sufficient distance, the Up c may be nloeed. and the poles) will 

 remain stationary. Aa aoon, however, aa o ia opened fg^fa 

 the pressure of o will drive the air back into the leeee ball 

 The npper pole will then fall, and at the eame time raise th 

 lower one at half the speed by means of the cord j the frffPtr 

 motion of the polea ia thus obtained. 



We have now to see bow the lamp ia made self -acting. Two 

 hollow bobbins, wound with ' -M-H wire, are placed aid* by 

 aide. (In the figure only one of theea, u, U MSA, the other 

 being hidden behind it) Two soft boo rods, I, fit loosely in- 

 side these, and are connected at the upper end by a eroes piece, 

 as shown in Fig. 36. This u suspended by a catgut thread from 

 an arc K, which ia fixed to one end of the lever K u. A screw 

 at the upper end of this arc allows the cord to be lengthened 

 or shortened at pleasure. At the other cad of the lever ia a 



i. . k I .. :. 



fastens to a 

 mall spiral 



prior * 

 and the 

 pivot B, on 

 which the 

 lever turns, 

 is a second 

 tap in the 

 tube, so ar- 

 ranged that 

 when the 

 lever ia in 



t:i.) ; M M : 

 indicated. 



Fig. 37. EXCAVATING BY LI.:.' :ii;i.' LIUUT. 



of the upper pole supplies the power required to move the carbons. 

 Fig. 35 will give a good general idea of the construction of the 

 apparatus ; the position of many of the parts is, however, 

 different from that occupied by them in the real lamp, the object 

 in the illustration being to render them all aa distinct as 

 possible. 



The india-rubber ball, A, is placed in a tin cylinder which it 

 just fits, and its mouth is connected with a tube terminating 

 in an open end, B ; a stopcock, c, is placed at a convenient 

 part of this tube. A flat weight, D, which carries the upper 

 pole, presses on the ball, and the carrier is kept vertical by 

 a tube fixed to the top of the instrument. The construction 

 of the upper pole is similar to that in the lamp already described, 

 and need not, therefore, be further explained ; the lower one 

 slips freely in its tube, and at the lower end of its carrier there 

 is a small brass pulley, F. To one side of the weight D is fixed 

 a small wire peg, which passes through a slit in the cylinder, 

 and has fastened to it one end of a cord. This cord passes 

 over the small fixed pulley K, then round th&b of the carrier 

 of tho lower polo F, and is fastened at the other end to a small 

 hook, o. 



The motion of the two poles will now bo easily understood. 

 When the lamp is to be used, an india-rubber ball of about the 

 same size as A is taken, the tube of it is slipped on tho open end 

 B, tho tap c is then opened, and the air from the ball pressed 

 into A. The immediate result of this is to raise the weight D 

 und the upper pole. In doing so, however, the wire peg rises also, 



I ia made for 

 9H the air into 

 I or out of the 

 1 ball A; but 

 aa soon aa 

 the end K 

 ia drawn 

 down it ia 

 closed. 



Now let 

 us trace the 

 course of 

 the electric 



current through tho whole apparatus. The positive wire ia 

 connected with the binding screw P; from this the current 

 passes by the cylinder to the upper pole, and thence to the 

 lower one ; it is not allowed, however, to pass into the instru- 

 ment, an insulating ring being placed at T. It travels, there- 

 fore, down the wire Q, round both the bobbins, and then by 

 another wire to N. 



When the lamp is to be lighted, the polea are first separated, 

 as already described, and the tap c closed ; the battery wires 

 are then connected with r and N respectively, and everything is 

 complete. We have now merely to open the tap c, when tJio 

 poles will commence to travel towards each other. Aa aoon 

 as they meet, the circuit will be completed, and the current will 

 pass round the bobbins ; the rods at I will then be drawn down 

 into them, and this will at once close the Up R, and pieient 

 any further motion of the poles. At the same time, by the 

 movement of the lever, the hook Q will be lowered, the cord 

 thereby slackened, and the lower polo will thus be separated 

 from the upper, so as to give the full effect of the light. The 

 hook o is made to slip along K L, and can be clamped in any 

 position by a small screw above it. so that in this way the play 

 of the lower pole may be altered according to the power of 

 the battery. 



If now the current becomes at all feeble, the apring * will draw 

 up tho rods a little way, and thna allow a small amount of an 

 to pass the Up B, until the poles come near enough to givs) the 

 full power again. A screw, o, ia provided by which the lower 



