Expiration of the Tesla Fundamental Polyphase Patents

The expiration this week of the three Tesla patents covering fundamentally the rotary-field type of electric motor marks the close of the first stage of one of the several epochal developments of the modern electrical era. It would be interesting to review the great rôle that the invention covered by these patents has played in electrical engineering and in industrial electrical progress, but what is probably at present uppermost in the minds of all interested directly in this branch, is the effect of the expiration of the 1888 Tesla patents toward opening up to the world at large a department of the industry which, for seventeen years, has been a monopoly under the Federal constitutional provision that so wisely grants this privilege as a reward for the labors of the inventor, and to encourage others through invention to add to the wealth and to the well-being of the country. As in the case of the termination of other close patent monopolies, there is unfortunately no clear understanding at the present moment as to the actual situation developed by the expiration of the 1888 Tesla patents. The litigation of the past has had relation only to two sets of polyphase patents — those just expired and the 1893 Tesla split-phase group — and little or no specific information is obtainable in regard to the situation now presented, even from those who have taken an active part in this litigation. To complicate the matter still further, it is announced by the owners of the patents just expired that more than a score of other patents relating to polyphase motors and polyphase operation are yet active, the inference being that it is by no means conceded that the polyphase art is opened wide by the expiration this week of the three 1888 Tesla patents.

Of the three patents which have just expired, the first has been defined by the courts as covering the apparatus by which the polyphase system is put in practice; the second as covering the electrical transmission of power by the method of operation described in the first patent, which patent also covers a different construction of motor; and the third as covering a specific construction of motor embodying the invention of the other two patents. In considering these three patents, the courts in various decisions have held them, broadly, to cover the system of producing power from an electric motor by means of a rotary field. It is, therefore, reasonable to assume that the expiration of these three patents releases to the world the principle of the rotary field. As to the split-phase patents, the consideration of their validity was so bound up with the rotary-field principle that their affirmation by the courts is now subject to challenge, which doubtless will be offered. While the rotary-field principle was invariably sustained in all litigation, the invention by Tesla of the split-field principle was denied by two courts of first instance, one of which was sustained on appeal and the other reversed.

Naturally, during the seventeen years' life of the patents just terminated, many subsidiary patents have been issued, which latter, owing to the controlling issue of the rotary field, have not figured in litigation or, at least have not been subject to interpretation by courts. To what extent this line of patents may narrow the effect of the termination of the life of the fundamental patents is a matter for judgment by those intimately interested, or for determination ultimately by the courts. If litigation with respect to these subsidiary and detail patents should ensue, it is to be hoped that during its course some principle may be established making a clear distinction between invention proper and improvement merely incident to the normal development of a basic invention. The criterion which the courts have in the past so freely applied — that lack of prior conception is a presumption in favor of real invention — is certainly of doubtful value when applied to an art closed to the world during its period of development.