Page:Popular Science Monthly Volume 43.djvu/753

Rh be grouped in various combinations. For the regular operation of the boats the cells are grouped in three divisions containing twenty-six cells each, arranged in series.

The batteries are charged for a run of ten to twelve hours, and are then recharged at the power station of the fleet in from five to seven hours. The launches run over a course of about three miles, at a speed of six miles an hour, and make landings at the principal buildings, all of which front upon the waterways.

To the engineer and to those who desire to know the trend of electrical development, the most interesting exhibit at the Fair will doubtless be the apparatus designed to show the long-distance transmission of power. Almost at the beginning of the modern electrical era, dreams were indulged in of the command which electricity was to give us of the natural sources of power. Marcel Deprez, at the Paris Exposition of 1881, had in operation a system of power transmission, and similar attempts have been made at every important exposition since, the most elaborate having been that at the Frankfort Exposition of two years ago. Of the importance of the economic transmission of power over long distances there can not be two opinions. The modern world has come to rest down upon an abundant and cheap supply of power in such a measure that without it civilization itself would go by the board. Statisticians have frequently shown that the coal supply, while large and ample for present needs, is not only exhaustible, but is being encroached upon at such a rate as to make its conservation a matter of grave concern. Electric transmission of power, by opening up to civilization the enormous supply of power of the waterfalls and running streams of the earth, will be able to postpone indefinitely the evil day that would be ushered in by the failure or material decrease of our fuel supply. To be of avail, however, such transmission must be economical, not only in the percentage of utilizable power sent through the line, but in the investment which must be made to realize it. So long as we were dependent upon the direct current, but little progress could be expected in this important problem. It has only been, therefore, in the last few years that the art was ripe for the taking up of this subject in a serious spirit, and with any hope of a real solution. The direct-current dynamo, handicapped with the commutator, is necessarily limited to supplying currents of relatively low voltage; the economic transmission of power demands the use of currents of small volume and very high pressure. This means small line conductors, and hence a relatively small investment. It means also a small loss in heating the line, since the heating power of the current varies as the square of the volume transmitted.