Page:Popular Science Monthly Volume 45.djvu/642

622 only one four-hundredth as much over a given wire as it would be if transmitted at fifty volts.

The advantage that alternating currents have over direct for long-distance transmission is that they may easily be transformed up or down—that is, their voltage at the generating end may be increased (at the expense, of course, of their amperage) and reduced at the consuming end. In point of fact, it is frequently and usually unnecessary to employ such devices at the generating end, for the reason that the generators themselves can work perfectly well at the high voltage requisite to transmit. The objection to using the same high voltage on the consuming machinery is simply that there is more danger of accident with numerous small motors scattered in various places and in the hands of unskilled persons than in a power station containing only two or three highly guarded machines attended by trained operatives.

With this fact of the possibility of generating currents of a voltage suitable for immediate transmission, it at first sight appears strange that direct-current transmission is not a more common thing than it is. The method of the so-called "motor transformer," "rotary transformer," or "dynamotor," might be adopted. A transmission plant working on this method would operate as follows: The power station would contain preferably several highly insulated direct-current generators, all of similar construction, for very high potential (four thousand volts would be easily obtained); these would run in series that is, each would add its voltage to that of the others, and there would preferably be a spare machine to substitute for any one of the others which might become injured. If four machines were in series, the resultant current would be put to line at, say, sixteen thousand volts, would be received at the other end by a number of motors, also in series, which in their turn would drive low potential dynamos supplying current for local use.

There are two objections to this as compared with alternating-current transmission: One is the fact that there has grown up a very tangible, we may almost call it, superstition against the use of high-voltage direct-current machines of large size among very many electricians. The reasons for this are not difficult to trace; prominent among them being the simple fact that no commercial application has ever yet required such machines. The only high-potential direct-current dynamos are those used for arc lighting, and on account of the great subdivision of arc-lighting circuits the units of generation are invariably small, at least by comparison with the ponderous machinery used in the Niagara Falls power plant.

There is no reason why they could not be made large (in point of fact, arc-lighting requirements are continually making demands