Tesla Electric Co. v. Scott

97 F. 588, 1899 U.S. App. LEXIS 3325
CourtU.S. Circuit Court for the District of Eastern Pennsylvania
DecidedSeptember 27, 1899
DocketNo. 28
StatusPublished
Cited by6 cases

This text of 97 F. 588 (Tesla Electric Co. v. Scott) is published on Counsel Stack Legal Research, covering U.S. Circuit Court for the District of Eastern Pennsylvania primary law. Counsel Stack provides free access to over 12 million legal documents including statutes, case law, regulations, and constitutions.

Bluebook
Tesla Electric Co. v. Scott, 97 F. 588, 1899 U.S. App. LEXIS 3325 (circtedpa 1899).

Opinion

McPHERSON, District Judge.

The defendants are charged with infringing letters patent Nos. 511,915 and 555,190, granted to the plaintiff as assignee of Nikola Tesla. The patents relate to electromagnetic motors operated by alternating currents of electricity, and the points in controversy cannot be understood without a brief preliminary consideration of certain properties manifested by the electric fluid. I condense the following account from the brief of plaintiff’s counsel, understanding that no objection is made by the defendants to the accuracy of the theory now to be repeated:

Suppose two coils of insulated wire, A and B, to be wound in opposite directions, and placed at two points on a soft iron bar, C. If a current of electricity be passed through either coil, or through both coils, the bar will become a magnet for the time being. If a current of given direction be passed through coil A alone, a north pole will be thus produced at one end of the bar, and a south pole at the other end. The same current passed through coil B alone will also produce north and south poles, but in reversed positions. If the same current be passed through both coils at the same time, a north pole will be produced in the bar midway between the coils. If the current through coil A is weaker than the current through coil B, the stronger current will have more influence upon the position of the pole, and will fix it nearer to A than to B. If the stronger current be shifted from A to B, the position of the pole will also be shifted to a point nearer to B than to A; and thus, by increasing [589]*589and decreasing from zero to maximum the relative strength of the currents through the respective coils, the pole may be made to travel backward and forward from one end of tbe bar to tbe other. If, therefore, a magnetic bar or armature, D, be suspended near the bar, and be free to move, it will inevitably follow the pole in this backward and forward movement. The bar need not be continuous, and need not be straight. It may be bent into the form of a ring. Soft iron forms a better path for the magnetic lines that llow from the coils than the air affords; bat the lines will pass through the air for some distance, and therefore the middle section of the bar may be removed, leaving only the sections that are surrounded by the coils, or the bar may be removed altogether. In either event the armature, D, will still be affected magnetically, while its movement may be less obstructed. The partial removal of the bar may therefore be an important physical advantage. The movement of the armature will dejiend upon the relative strength of the magnetizing influences of the coils, and upon the forces that may he set in motion by these influences.

If, therefore, the pole is to shift uniformly, and is thus to produce a uniform magnetic attraction, the electric current passing through each coil must vary in strength, and must vary uniformly. There must be a definite relation between the two currents, and this relation must be constant in all the changes in strength that the currents undergo. Tesla discovered — and the discovery has been of vast importance — how to use alternating currents of electricity so as to bring about this uniform shifting of the poles or attractive forces in a motor, and thus to cause the rotation of an armature, and the consequent transmission of power.

A continuous current of electricity flows in one direction only, and in its course does not vary much in strength. An alternating 'current flows backward and forward, and in its course varies in strength four times between zero and maximum. It begins at zero, increases in a given direction to maximum, decreases again to zero, increases to maximum, but now in the opposite direction, and finally decreases once more to zero. Tbe following curve will serve to illustrate what takes place:

The application of alternating currents in the operation of motors will he best shown by examining another of Tesla’s patents, Ho. 381,968. Mg. 3 of this patent shows a generator of alternating currents, and a motor connected therewith. In this motor the soft iron bar, C, is bent into tbe shape of a ring, around which are wound two pairs of magnetizing coils, A and B, arranged alternately. The armature, D, is centrally mounted within the ring, and is free to [590]*590revolve upon this central axis. If a current be passed through coil A alone, a horizontal line of polarization will be developed in the core, C. A similar current through coil B alone will develop a vertical line of polarization, while currents of equal stréngth passed through both sets of coils at the same time will produce a resultant line of polarization that will be diagonal in direction, midway be: tween the horizontal and vertical lines. If the two currents be applied at the same time, but varying in strength and direction, the poles will travel around the ring, and the shifting magnetic attraction, or rotating held, will draw the armature after it. The necessary variations in the strength and direction of the two currents are produced in the generator. What takes place there is thus detailed in the account I have been following:

“E and F, in said generator, are two field-magnet poles; G, a cylindrical armature mounted to rotate between them; and H, K, two coils wound on the armature at right angles to each other.
“It is well known that, when any coil is moved in a magnetic field so as to cut at right angles the lines of force extending between the poles, a current is developed in the coil, which depends for direction upon the direction of movement of the coil with respect to the magnetic lines, and for strength or potential upon the number of lines cut per unit of time.
“In any given generator, therefore, such as that shown in Fig. 3, in which the magnetic lines may be assumed to extend across, in parallelism, from one pole to the other, an armature coil, H, moving through such a field in a circular path, will, at one instant of time, cut no lines of force, since its direction is parallel to the direction of the lines. At such instant the coil is developing no current, and is said to be in the neutral position.
“At another point in its path, 90 degrees from the neutral point, the coil Is cutting the lines of force at right angles. It is then said to be in the maximum position, because its direction is such as to cut the greatest number of lines per unit of time, and therefore to produce the maximum current. At every intermediate point in its path, from the neutral to the maximum points, it is evident that the coil will cut a gradually increasing number of lines of force in a given angular movement, and will in consequence develop a current which rises from zero to maximum.
“The coil, in passing beyond the maximum point, develops gradually less current until it reaches a point 180 degrees from the start, where it is again said to be at the neutral point, and its current is nil. In moving through the next 180 degrees of arc, the action above described is repeated, but the direction of current is reversed, since the coil is moving in the opposite direction across the lines of force. When the coil has returned to the assumed starting point, it has undergone a complete cycle of changes, and a current has been developed which, in potential and direction, corresponds throughout to the position and movement of such coil.

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Bluebook (online)
97 F. 588, 1899 U.S. App. LEXIS 3325, Counsel Stack Legal Research, https://law.counselstack.com/opinion/tesla-electric-co-v-scott-circtedpa-1899.