VOLTAIC ELECTRICITY. 



borotn' 





i: ; whioh shows that the greatest 

 5 7 



power JH obtained when they are connected in two* or three*. 

 TheM calculations the student should verify for hioiMolf. In 

 battery so as to ul>tuiu from it the greatest power 



v given work, wo should BO join the elumunU that the 

 number arranged in series may be u nearly a* possible a moan 

 between tho inunlu-r <( the cell* and the number which expresses 

 the proportion between B and r. Thuj, if r = 6 B, and there 

 are twenty rolln, the belt result* will be obtained by arranging 

 them in two Hories of ten each. 



From thin we see that wo need Rome mode of ascertain- 

 ing tho resistance which in offered by any conductor to tho flow 

 of tho current along it, and wo Khali shortly learn 

 the manner in whioh this is accomplished. Before, 

 however, proceeding to this, we must say a little 

 more about tho mode of conducting tho current from 

 place to placo. In f notional electricity we saw tho 

 absolute m-ed of providing somo very good insulator 

 to confine it ; tho voltaic current, however, has far 

 less tension, and therefore manifests much less 

 ucy to escape. For most ordinary purposes 

 common uncovered copper wire may be used, and 

 but littlo of the current will be dissipated ; still, 

 when it is but faint, and has to bo conveyed to a 

 considerable distance, insulated wire must be em- 

 ployed. This usually consists of copper wire, 

 covered by one or more layers of gutta peroha, by 

 which means the wire may be handled, or even rested on metal 

 supports, without any of tho fluid escaping. In telegraph lines 

 this mode of insulation would be found very expensive, and plain 

 wire, usually made of galvanised iron and suspended from porce- 

 lain or ebonite insulators, is therefore employed. The construc- 

 tion of these will be fully explained when wo come to treat of the 

 electric telegraph. 



In order for any effect to be produced the circuit must be 

 closed ; that is, there must bo a connection between tho positive 

 and negative poles. If this communication be interrupted in any 

 part of the circuit all action will at once cease, and if in any 

 part the surfaces of two adjacent conductors are rusted or dirty, 

 so that they do not come into close contact, the whole force of 

 the current will be reduced. Sometimes the power of a largo 

 battery is very much diminished by a faulty connection of this 

 kind; great care is therefore requisite to guard against this 

 mishap. 



On account of this necessity for the circuit being closed, a 

 return wire used always to be employed to bring tho current 

 back when it had been conveyed to a great distance as, for 

 example, in the case of a telegraph line. It was soon found, 

 however, that this might be dispensed with, and that the earth 

 would act the part of the return wire. One of the wires, on 

 leaving the instrument, is put in metallic 

 communication with a large plate of 

 metal usually buried in charcoal, so as 

 to ensure a free passage for the current, 

 and a similar plate is buried at tho 

 other station and connected with one of 

 the poles of the battery ; and this ar- 

 rangement answers most perfectly, the 

 time occupied by the passage o.* the 

 current along the line wire and bock by 

 the earth being quite inappreciable. 



A metallic connection to ordinary gas or water pipes will 

 answer the purpose equally well. The wire should, however, bo 

 soldered to tho pipe, as otherwise rust may form at the point of 

 contact. 



Some point to it as a most wonderful fact that tho electricity 

 should thus find its own way back again to the battery from 

 which it started, even though many others are in the same placo ; 

 while others look upon the earth as merely acting tho part of a 

 reservoir for the current. Tho former is probably the correct 

 explanation. But it is immaterial which theory is adopted, aa 

 the fact itself is tho important thing to observe ; and in the 

 course of our lessons we shall moot with many other things 

 equally strange and wonderful. 



In frictional electricity wo found that tho spark always picked 

 out for itself the shortest path ; a similar effect is observed with 

 voltaic electricity, not, however, to tho same extent, as, if there 



are two path* open, one of which in not very much aborter the* 

 the other, the current will be divided, and a portion will trf d 

 along each. The greater part, however, will pa** along the 

 shorter road ; and if it be much the shorter, and ooapo*ed of 

 larger or better conductors, *oaroely any will paes aloof the 

 other path. 



Advantage is very often taken of thi* fact in the 



ge u very often taken of this 



of various pieces of apparatus, a short etrcuil, as it is termed. 

 being made so ae to divert the current from certain portions of 

 the instrument when required. 



It may be well just to guard the student against las|iii(i|. 



ieml fluid 



from tho one of tho word current, that there i* any 

 pacing along the wires in the way in which water is conveyed 

 through metal pipe*; there u merely a transfer at 

 force to any required part of the circuit. 



Wo must now go back a little, and try to **cer- 

 tain the mode in whioh the reeistanoe whioh is 

 offered to the passage of the current along any 

 conductor may be measured. This question is, as 

 was seen, an important one when we are employing 

 Ohm's laws, as by it the internal and external re- 

 sistances (B and r) may be measured. It is, how. 

 over, of much greater importance in many other 

 ways. In constructing long submarine cables it is 

 important to choose a conductor which shall offer 

 as little resistance as possible to the paeeage of 

 the current, and we want some means of ssocr 

 taining this ; this means is afforded by the instru- 

 ments we arc about t > describe. Another important end 

 served by them is to ascertain the position of a fault or injury 

 in a long lino, and thus to know where to send a staff of men 

 to repair it. 



Tho principle on which many of these instrument* <P*p*ndff r is 

 that when a current passes over an ordinary magnetised needle 

 it deflects it to tho right or the left, according to the direction 

 in which tho current is passing, and the amount of its deviation 

 indicates the intensity of the current. To render the ii 



26. 



more delicate, tho wire is usually made to pass several 

 round the noodle, and the instrument thus formed is 

 galvanometer. The onds of the wires are fixed to binding screw* 

 at tho sides, as shown in Fig. 26. The principle of this instru- 

 ment, and tho various modifications adopted to increase it* 

 sensitiveness, will be described in their place ; the student can, 

 however, easily construct one for himself, and will find it very 

 useful. Take a circular disc of hard wood four or five inches in 

 diameter, and in the centre place a small pivot on which an 

 ordinary compass needle may be balanced. Now take a strip of 

 cardboard about an inch wide, and bend it round so as to form an 

 oblong tube about three inches long and three-quarters of an inch 

 high. A piece of wire should be wound very evenly round this 

 about twenty or thirty times, taking care to make the coil* lie 

 parallel to one another, and to leave out 

 the ends. This coil is now fixed down 

 upon the disc, so that the point in the 

 centre may come through the middle of 

 one side, and the ends are then connected 

 with the binding screws at the side*. 

 The neatest way of doing this is to let 

 them pass through holes in the board 

 to the under side, then along in little 

 grooves cut for them, and up again through 

 the board to the under part of the i 



If this galvanometer be now placed so that the needle reste 

 parallel with the coil, and the battery wires are connected with 

 the binding screws, the needle will at once diverge, and on 

 reversing the battery wires the north polo will project on the 

 other side of the coil. If, however, we send two equal current* 

 through the galvanometer in opposite directions, they will exactly 

 neutralise one another, and the needle will remain at rest. 



On this principle the instrument known as Wheatstone'e 

 Bridge acts ; its construction will easily be understood by re- 

 ference to tho illustration (Fig. 27). A and B are binding 

 screws, fixed near tho ends of a piece of wood, in the centre 

 of which is placed a galvanometer (o). Along one side of the 

 board are placed two pairs of binding screws (, r and H, K) ; 

 these are one or two inches apart, c and o are the hmitinf; 

 screws of the galvanometer. 



Connections are then made between the binding screw* by 



