488

INVENTIONS OF NIKOLA TESLA.

practice by combining an electric generator with his oscillator.He pointed out what conditions must be observed in order thatthe period of vibration of the electrical system might not disturbthe mechanical oscillation in such a way as to alter the periodicity,but merely to shorten the stroke. He combines a condenserwith a self-induction, and gives to the electrical system the sameperiod as that at which the machine itself oscillates, so that bothtogether then fall in step and electrical and mechanical resonanceis obtained, and maintained absolutely unvaried.

Hext he showed a model of a motor with delicate wheel work,which was driven by these currents at a constant speed, no mat-ter what the air pressure applied was, so that this motor couldbe employed as a clock. He also showed a clock so constructedthat it could be attached to one of the oscillators, and wouldkeep absolutely correct time. Another curious and interestingfeature which Mr. Tesla pointed out was that, instead of con-trolling the motion of the reciprocating piston by means of aspring, so as to obtain isochronous vibration, he was actually ableto control the mechanical motion by the natural vibration of theelectro-magnetic system, and he said that the case was a verysimple one, and was quite analogous to that of a pendulum.Thus, supposing we had a pendulum of great weight, preferably,which would be maintained in vibration by force, periodicallyapplied ; now that force, no matter how it might vary, althoughit would oscillate the pendulum, would have no control over itsperiod.

Mr. Tesla also described a very interesting phenomenon whichhe illustrated by an experiment. By means of this new appara-tus, he is able to produce an alternating current in which thee. m. f. of the impulses in one direction preponderates over thatof those in the other, so that there is produced the effect of adirect current. In fact he expressed the hope that these cur-rents would be capable of application in many instances, servingas direct currents. The principle involved in this preponderat-ing e. m. f. he explains in this way: Suppose a conductor ismoved into the magnetic field and then suddenly withdrawn. Ifthe current is not retarded, then the work performed will be amere fractional one; but if the current is retarded, then themagnetic field acts as a spring;. Imagine that the motion of theconductor is arrested by the current generated, and that at theinstant when it stops to move into the field, there is still the