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UNITED STATESPATENT OFFICE
NIKOLATESLA, OF NEW YORK, N.Y.
ELECTRICGENERATOR.
SPECIFICATIONforming part of Letters Patent No. 511,915 dated January 2, 1894.
Applicationfiled August 19,1893. Serial No. 433,562 (No model.)

To all whom it may be concern:
Be it known that I, NIKOLA TESLA, a citizen of the United States, residing at NewYork, in the country and State of New York, have invented certain new anduseful improvements in Electric Generators, of which the following is aspecification, reference being had to the drawings accompanying and forming apart of the same.
In an application of even date herewith, Serial No. 483,563, I have shown anddescribed a form of engine invented by me, which under the influence of an appliedforce such as the elastic tension of stream or a gas under pressure, yields anoscillation of constant period.

It is a well known mechanical principle that if a spring possessing a sensibleinertia be brought under tension, as by being stretched, and then freed, itwill perform vibrations which are isochronous, and as to period, in the main,dependent upon the rigidity of the spring, and its own inertia or that of thesystem of which it may form an immediate part. This is known to be true in allcases where the force that tends to bring the spring or movable system into agiven position is proportionate to the displacement.

In the construction of my engine abovereferred to I have followed and applied this principle, that is to say, Iemploy a cylinder and a piston which in any suitable manner I maintain inreciprocation by steam or gas under pressure. To the moving piston or to thecylinder, in case the latter reciprocate and the piston remain stationary, aspring is connected so as to be maintained in vibration thereby, and whatevermay be the inertia of the piston or of the moving system and the rigidity ofthe spring relatively to each other, provided, the practical limits withinwhich the low holds true that the forces which tend to bring the moving systemto a given position are proportionate to the displacement, are not exceeded,the impulses of the power impelled piston and the natural vibrations of thespring will always correspond in direction and coincide in time. In the case ofthe engine referred to, the ports are so arranged that the movement of thepiston within the cylinder in either direction ceases when the force tending toimpel it and the momentum which it has acquired are counterbalanced by theincreasing pressure of the steam or compressed air in that end of cylindertoward which it is moving, and as in its movement the piston has shut off at agiven point, the pressure that impelled it and established the pressure thattends to return it, it is then impelled in the opposite direction, and thisaction is continued as long as the requisite pressure is applied. The length ofthe stroke will vary with the pressure, but the rate or period of reciprocationis no more dependent upon the pressure applied to drive the piston, than wouldbe the period of oscillation of a pendulum permanently maintained in vibration,upon the force which periodically impels it, the effects of variations in suchforce being merely to produce corresponding variations in the length of strokeor amplitude of vibration respectively.

In practice I have found that the best resultsare secured by the employment of an air spring, that is compressed and rarefiedby the movements of the piston, and in order to secure a spring of constantrigidity I prefer to employ a separate chamber or cylinder containing air atthe normal atmospheric pressure, although it might be at any other pressure,and in which works a plunger connected with or carried by the piston rod. Themain reason why no engine heretofore has been capable of producing result s ofthis nature is that it has been customary to connect with the reciprocatingparts a heavy fly-wheel or some equivalent rotary system of relatively verygreat inertia, or in other cases where no rotary system was employed, as incertain reciprocating engines or tools, no regard has been paid to theobtainment of the conditions essential to the end which I have in view, norwould the pressure of such conditions in said devices appear to result in anyspecial advantage.

Such an engine as I have described affordsa means for accomplishing a result heretofore unattained, the continuedproduction of electric currents of constant period, by imparting the movementsof the piston to a core or a coil in a magnetic filed. It should be statedhowever, that is applying the engine for this purpose certain conditions are encounteredthat should be taken into consideration in order to satisfactorily secure thedesired result. When a conductor is moved in a magnetic field and a currentcaused to circulate therein, the electro-magnetic reaction between in and thefield, might disturb the mechanical oscillation to such an extend as to throwit out of isochronism. This, for instance, might occur when theelectro-magnetic reaction is very great in comparison to the power of theengine, and there is retardation of the current so that the electro-magneticreaction might have an effect similar to that which would result from avariation of the tension of the spring, but if the circuit of the generator beso adjusted that the phases of the electromotive force and current coincide intime, that is to say, when the current is not retarded, then the generatordriven by the engine acts merely as a frictional resistance and will not, as arule, alter the period of the mechanical vibration, although it may vary its amplitude.This condition may be readily secured by properly proportioning theself-induction and capacity of the circuit including the generator. I havehowever, observed the further fact in connection with the use of such enginesas a means for running a generator, that it is advantageous that the period ofthe engine and the natural period of electrical vibration of the generatorshould be the same, as in such case the best conditions for electricalresonance are established and the possibility of disturbing the period ofmechanical vibrations is reduced to a minimum. I have found that even if thetheoretical conditions necessary for maintaining a constant period in theengine itself are not exactly maintained, still the engine itself are notexactly maintained, still the engine and generator combined will vibrate at aconstant period. For example, if instead of using in the engine an independentcylinder and plunger, as an air spring of practically constant rigidity, Icause the piston to impinge upon air cushions at the ends of its own cylinder,although the rigidity of such cushions or springs might be considerablyaffected and varied by the variations of pressure within the cylinder, still bycombining with such an engine a generator which has a period of its ownapproximately that of the engine, constant vibration may be maintained eventhrough a considerable range of varying pressure, owing to the controllingaction of the electro-magnetic system. I have even found that under certainconditions the influence of the electro-magnetic system may be made so great asto entirely control the period of the mechanical vibration once started, is notsufficient to change its rate. So, for the sake of illustration, if a pendulumis started in vibration, and a small force applied periodically in the properdirection to maintain it in motion, this force would have no substantialcontrol over the period of the oscillation, unless the inertia of the pendulumbe small in comparison to the impelling force, and this would be true no matterthrough what fraction of the period the force may be applied. In the case underconsideration the engine is merely an agent for maintaining the vibration oncestarted, although it will be understood that this does not preclude theperformance of useful work that would simply result in a shortening of thestroke. My invention, therefore, involves the combination of a piston free toreciprocate under the influence of stream or a gas under pressure and themovable element of an electric generator that is in direct mechanical connectionwith the piston, and it is more especially the object of my invention to securefrom such combination electric currents of a constant period. In the attainmentof this object I have found it preferable to construct the engine so that it ofitself controls the period, but as I have stated before, I may so modify theelements of the combination that the electro-magnetic system may exert apartial or even complete control of the period.

In illustration of the manner in which theinvention is carried out I now refer to the accompanying drawings.
Figure1 is a central sectional view of an engine and generator embodying theinvention. Figure 2 is a modification of the same.
Referringto Figure 1 A is the main cylinder in which works a piston B. Inlet ports C-Cpass through the sides of the cylinder opening at the middle portion thereofand on opposite sides. Exhaust ports DD extend through the walls of thecylinder and are formed with branches that open into the interior of thecylinder on each side of the inlet ports and on opposite sides of the cylinder.The piston B is formed with two circumferential grooves E F that communicatethrough openings G in the piston with the cylinder on opposite sides of saidpiston respectively.

The particular construction of thecylinder, the piston and the ports controlling it may be very much varied, andis not in itself material, except that in the special case now underconsideration it is desirable that all the ports, and more especially theexhaust ports should be made very much larger than is usually the case so thatno force due to the action of the stream or compressed air will tend to retardor affect the return of the piston in either direction. The piston B is securedto a piston rod H that works in suitable stuffing boxes in the heads of thecylinder A. This rod is prolonged on one side and extends through bearings V ina cylinder I suitably mounted or supported in line with the first, and withinwhich is a disk or plunger J carried by the rod H. The cylinder I, is without portsof any kind and is airtight except as a small leakage may occur through thebearings V, which experience has shown need not be fitted with any veryconsiderable accuracy. The cylinder I, is surrounded by a jacket K that leavesan open space or chamber around it. The bearings V in the cylinder I, extendthrough the jacket K to the outside air and the chamber between the cylinderand jacket is made steam of airtight as by suitable packing. The main supplypipe L for steam or compressed air leads into this chamber, and the two pipesthat lead to the cylinder A run from the said chamber, oil cups M beingconveniently arranged to deliver oil into the said pipes for lubricating thepiston. In the particular form of engine shown, the jacket K that contains thecylinder I, is provided with a flange N by which it is screwed to the end ofthe cylinder A. A small chamber O is thus formed which has air vents P in itssides and drip pipes Q leading out from it through which the oil which collectsin its is carried off.

To explain now the operation of the enginedescribed, in the position of the parts shown, or when the piston is at themiddle point of its stroke, the plunger J is at the center of the cylinder Iand the air on both sides of the same is at the normal pressure of the outsideatmosphere. If a source of steam or compressed air were then connected to theinlet ports C-C of the cylinder A, and a movement be imparted to the piston asby a sudden blow, the latter is caused to reciprocate in a manner wellunderstood. The movements of the piston compress and rarefy the air in thecylinder I at opposite ends of the same alternately. A forward strokecompresses the air ahead of the plunger J that acts as a spring to return it.Similarly on the backstroke the air is compressed on the opposite side of theplunger J and tends to drive it forward. The compressions of air in thecylinder I, and the consequent loss of energy due mainly to the imperfectelasticity of the air, give rise to a very considerable amount of heat. This heatI utilize by conducting the steam or compressed air to the engine cylinderthrough the chamber formed by the jacket surrounding the air-spring cylinder.The heat thus taken up and used to raise the temperature of the steam or airacting upon the piston is availed of to increase the efficiency of the engine.In any given engine of this kind the normal pressure will produce a stroke ofdetermined length, and this will be increased or diminished according to theincreases of pressure above or the reduction of pressure below the normal.

In constructing the apparatus properallowance is made for a variation in the length of stroke by giving to theconfining cylinder I of the air spring properly determined dimensions. Thegreater the pressure upon the piston, the higher the degree of compression ofthe air spring. And the consequent counteraction forces upon the plunger. Therate or period of reciprocation of the piston, however, is mainly determined asdescribed above by the rigidity of the air spring and the inertia of the movingsystem, and any period of oscillation within very wide limits may be secured byproperly portioning these factors, as by varying the dimensions of the airchamber which is equivalent to varying the rigidity of the spring, or by adjustingthe weight of the moving parts. These conditions are all readily determinable,and an engine constructed as herein described may be made to follow theprinciple of operation above stated and maintain a perfectly uniform periodthrough very wide limits of pressure.

The pressure of the air confined in thecylinder when the plunger I is in its central position will always be practicallythat of the surrounding atmosphere, for while the cylinder is so constructed asnot to permit such sudden escape of air as to sensibly impair or modify theaction of the air spring there will still be a slow leakage of air into or outof it around the piston rod according to the pressure therein. So that thepressure of the air on opposite sides of the plunger will always tend to remainat that of the outside atmosphere.
To the piston rod H is secured a conductoror coil of wire D’ which by the movements of the piston is oscillated in themagnetic field produced by two magnets B’B’ which may be permanent magnets orenergized by coils C’C’ connected with a source of continuous current E’. Themovement of the coil D across the lines of force established by the magnetsgives rise to alternating currents in the coil. Theses currents, if the periodof mechanical oscillation were constant will be of constant period, and may beutilized for any purpose desired.
In the case under consideration it is assumed as a necessary condition that theinertia of the movable element of the generator and the electro-magnetic reactionthat it exerts will not be of such character as to materially disturb theaction of the engine.

Figure 2 is an example of a combination in whichthe engine is not of itself capable of determining entirely the period ofoscillation, but in which the generator contributes to this end. In this figurethe engine is same as Figure 1. The exterior air spring is however omitted andthe air spaces at he ends of the cylinder A relied on for accomplishing thesame purpose. As the pressure in these spaces is liable to variations fromvariations in the steam or gas used in impelling the piston they might affectthe period of oscillation, and the conditions are not as stable and certain asin the case of an engine constructed as in Figure 1. But if the natural periodof vibration of the elastic system were made to approximately accord with theaverage period of the engine such tendencies to variation are very largelyovercome and the engine will preserve its period even through a considerablerange of variations of pressure. The generator in this case is composed of amagnetic casing F’ in which a laminated core G’ secured to the piston rod H iscaused to vibrate. Surrounding the plunger are two exciting coils C’C’, and oneor more induced coils D’D’. The coils C’C’ are connected with a generator ofcontinuous currents E’ and are wound to produce consequent poles in the core G’.Any movement of the latter will therefore shift the lines of force throughcoils D’D’ and produce currents therein.