Page:Popular Science Monthly Volume 22.djvu/68

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THE POPULAR SCIENCE MONTHLY.

ing influence caused by charge in under-ground or submarine conductors. These conditions are particularly favorable to under-ground line wires, which possess other important advantages over the still prevailing over-ground system, in that they are unaffected by atmospheric electricity, or by snow-storms and heavy gales, which at not very rare intervals of time put us back to pre-telegraphic days, when the letter carrier was our swiftest messenger.

The under-ground system of telegraphs, first introduced into Germany by Werner Siemens in the years 1847-'48, had to yield for a time to the over-ground system owing to technical difficulties, but it has been again resorted to within the last four years, and multiple land cables of solid construction now connect all the important towns of that country. The first cost of such a system is no doubt considerable (being about £38 per kilometre of conductor as against £8 10s. the cost of land lines); but, as the under-ground wires are exempt from frequent repairs and renewals, and as they insure continuity of service, they are decidedly the cheaper and better in the end. The experience afforded by the early introduction of the under-ground system in Germany was not, however, without its beneficial results, as it brought to light the phenomena of lateral induction, and of faults in the insulating coating, matters which had to be understood before submarine telegraphy could be attempted with any reasonable prospect of success.

Regarding the transmission of power to a distance, the electric current has now entered the lists in competition with compressed air, the hydraulic accumulator, and the quick-running rope as used at Schaffhausen to utilize the power of the Rhine-fall. The transformation of electrical into mechanical energy can be accomplished with no further loss than is due to such incidental causes as friction and the heating of wires; these in a properly designed dynamo-electric machine do not exceed 10 per cent, as shown by Dr. John Hopkinson, and, judging from recent experiments of my own, a still nearer approach to ultimate perfection is attainable. Adhering, however, to Dr. Hopkinson's determination for safety's sake, and assuming the same percentage in reconverting the current into mechanical effect, a total loss of 19 per cent results. To this loss must be added that through electrical resistance in the connecting line wires, which depends upon their length and conductivity, and that due to heating by friction of the working parts of the machine. Taking these as being equal to the internal losses incurred in the double process of conversion, there remains a useful effect of 100 ${\displaystyle -}$ 38 ${\displaystyle =}$ 62 per cent, attainable at a distance, which agrees with experimental results, although in actual practice it would not be safe at present to expect more than 50 per cent of ultimate useful effect, to allow for all mechanical losses.

In using compressed air or water for the transmission of power, the loss can not be taken at less than 50 per cent, and as it depends upon fluid resistance it increases with distance more rapidly than in the case