ELECTRIC WAVE 



2850 



ELECTRO- BALLISTICS 



into the conductors for transmis- 

 sion may have to be " stepped 

 down " at convenient points, and 

 here the character of the current, 

 as well as the strength, will have 

 to be determined, that is whether 

 continuous or direct, or alternating. 

 Direct or continuous current, which 

 has the advantage of simplicity, 

 cannot be stepped down directly 

 without the use of rotating ma- 

 chinery. In large systems this may 

 be accomplished by the aid of sub- 

 stations to which high-tension cur- 

 rent is transmitted from the central 

 station, to be there let down by 

 mechanical transformers to suit the 

 local needs. Alternating current, 

 on the other hand, may be stepped 

 down by the use of static trans- 

 formers, which are automatic and 

 require nothing more than a street 

 box for their installation. 



The Hopkinson System 

 An ingenious method of meeting 

 the requirements where direct cur- 

 rent is employed was devised 

 by Dr. John Hopkinson, and is. 

 known as the three-wire system of 

 distribution. This is illustrated 

 in the appended diagram. A and B 

 are two dynamos coupled in series, 

 that is, the current from A passes 

 through B. CD are two mains or 

 conductors constituting a primary 



Electric Power. System of trans- 

 mission devised by Dr. John Hop- 

 kinson. See text 



circuit. If the voltage of each 

 dynamo is 100, the current put 

 into the main C will be at 200 volts : 

 that is, the potential difference be- 

 tween C and D. But the current 

 from A dynamo is going into the 

 third wire E at 100 volts, and the 

 potential difference between E and 

 D will therefore only, bo 100, which 

 will also be the difference between 

 E and C ; and consequently any 

 lamps between E and D or E and 

 C will be receiving current only at 

 that voltage. A further sub- 

 division by means of additional 

 wires is sometimes arranged. 



Except for tramlines, where the 

 current is mostly carried by the 

 familiar overhead lines, local trans- 

 mission and distribution is carried 

 almost universally in Great Bri- 

 tain by conductors placed under- 

 ground. The conductors may be 

 simply wire or cables thoroughly 

 insulated and protected against 

 damp or mechanical injury, and 

 laid in the ground under the foot- 

 path. In more elaborate schemes, 



groups of conduits composed of 

 earthenware pipes are prepared. 



The pipes a,re carefully cemented 

 together to exclude water, and a 

 number are left empty for future 

 use. At suitable points the con- 

 duits are interrupted and provision 

 is made by which access can be got 

 down to them. The cables are 

 pulled through these conduits from 

 point to point, and in order that 

 they may stand the severe usage 

 to which they are subjected in 

 being drawn through the conduits, 

 they are encased in lead. 



Apart from the main transmis- 

 sion conductors, there will usually 

 be laid supplementary conductors 

 known as feeders. These convey 

 current to some more or less distant 

 point of a main conductor, and 

 compensate for whatever may 

 have been taken from the main up 

 to that point by distributors. In 

 large systems where extended 

 areas are to be served, and a great 

 variety of demands have to be 

 satisfied, the transmission and dis- 

 tributing network may become 

 very complex. To meet such cases, 

 alternating current at high tension, 

 ranging from 1,000 to 6,000 volts, 

 is being employed at several 

 stations in this country. At one 

 London station the current, alter- 

 nating, is generated and trans- 

 mitted to sub-stations at 10,000 

 volts ; at the sub-stations it is 

 stepped down by motor generators 

 (see Motor, Electric) to 400 volts 

 continuous current, which is sup- 

 plied to users for power at that ten- 

 sion, and, by means of the three- 

 wire system, to users of light at 

 200 volts. 



Long Distance Transmission 



For long distance transmission 

 alternating current is almost in- 

 variably employed, because of the 

 facility' with which it can be 

 lowered by the aid of stationary 

 transformers ; while the current is 

 usually three-phase on account of 

 the economy of copper required for 

 transmission. The most remark- 

 able transmission system in the 

 world to-day is in the state of 

 California, U.S.A., where current 

 generated by water power at 

 Big Creek is transmitted to Los 

 Angeles, a distance of 240 m. 

 From the great stations of Niagara, 

 current is transmitted locally at 

 2,250 volts, but is sent into Buffalo 

 16 miles away at 11,000 volts. A 

 remarkable system is in operation 

 in the south of France, where con- 

 tinuous current is used at 58,000 

 volts. See Current, Electric ; 

 Dynamo; Volt. 



Electric Wave. In an insu- 

 lated conductor carrying a charge 

 of electricity, the charge will be dis- 

 tributed hi a regular manner over 



its surface, and if another con- 

 ductor be brought near it the dis. 

 tribution of the charge will be al 

 tered, say negative at the one end, 

 positive at the other. If now the 

 second conductor be suddenly re- 

 moved the original charge will re- 

 turn to its former distribution, but 

 not at once. Before the original 

 condition is reached the charge will 

 oscillate along the conductor, posi- 

 tive and negative each rushing 

 from one end to the other and then 

 back again. 



These oscillations are extremely 

 rapid and quickly die away. 

 This movement suggests that of 

 a wave, but it is not strictly of 

 that character. But if two con- 

 ductors be discharged by a spark 

 there will again be oscillations set 

 up between the terminals of the 

 conductors, with this difference 

 the charge thus liberated sets up 

 disturbances in the surrounding 

 medium which are in the nature of 

 true waves and go on travelling 

 through space. These waves have 

 been found to -possess all the 

 optical properties of the waves of 

 light, and can be reflected, re- 

 fracted, and polarised. These facts 

 were demonstrated in 1888 by 

 Heinrich R. Hertz, who succeeded 

 in producing electro - magnetic 

 waves by means of " oscillators " 

 in such a way as to permit him to 

 trace their propagation throiigh 

 space, which he found went on at 

 the same velocity as that of light. 

 See Light ; Wireless. 



Electro - Ballistics . Deter- 

 mination of the velocity of projec- 

 tiles fired from guns by electrical 

 methods, the success of which de- 

 pends upon the fact that the time 

 taken for an electric current to 

 travel along a wire over any such 

 distances as those covered by the 

 range of even the most powerful 

 guns, is inappreciable. 



The appliances used consist of 

 frames of light' wire mesh elec- 

 trically connected with a chrono- 

 graph. One such frame is placed 

 near the muzzle of the gun that is to 

 be tested, as close up as may be 

 so long as it is far enough away to 

 be unaffected by the blast of the 

 discharge. The frame is placed in 

 the line of flight of the projectile ; 

 the resistance offered by the wire 

 mesh to the projectile is negligible. 

 A similar frame is placed at a 

 measured distance from the gun 

 also in the line of flight ; or a series 

 may be placed at measured inter- 

 vals. When the gun is fired the 

 projectile crashes through the wire 

 mesh, the electrical arrangements 

 come instantly into action and the 

 exact moment is indicated by the 

 chronograph. It is only necessary 

 to compare the times recorded by 



