The result will be the time average of the current, if during the interval the current has varied.

In determining by this method the constant of an instrument the current should be kept as nearly constant as possible, and the readings of the instrument taken at frequent observed intervals of time.  These observations give a curve from which the reading corresponding to the mean current (time average of the current) can be found.  The current, as calculated by the voltameter, corresponds to this reading.

* * * * *

THE TWO OR THREE PHASE ALTERNATING CURRENT SYSTEMS.

By CARL HERING.

The occasion of the transmission of power from Lauffen to Frankfort has brought to the notice of the profession more than ever before the two or three phase alternating current system, described as early as 1887-88 by various electricians, among whom are Tesla, Bradley, Haselwander and others.  As to who first invented it, we have nothing to say here, but though known for some years it has not until quite recently been of any great importance in practice.

Within the last few years, however, Mr. M. Von Dolivo-Dobrowolsky, electrical engineer of the Allgemeine Elektricitats Gesellschaft, of Berlin, has occupied himself with these currents.  His success with motors run with such currents was the origin of the present great transmission of power exhibit at Frankfort, the greatest transmission ever attempted.  His investigation in this new sphere, and his ability to master the subject from a theoretical or mathematical standpoint, has led him to find the objections, the theoretically best conditions, etc.  This, together with his ingenuity, has led him to devise an entirely new and very ingenious modification, which will no doubt have a very great effect on the development of alternating current motors.

It is doubtless well known that if, as in Fig. 1, a Gramme ring armature is connected to leads at four points as shown and a magnet is revolved inside of it (or if the ring is revolved in a magnetic field and the current led off by contact rings instead of a commutator), there will be two alternating currents generated, which will differ from each other in their phases only.  When one is at a maximum the other is zero.  When such a double current is sent into a similarly constructed motor it will produce or generate what might be called a rotary field, which is shown diagrammatically in the six successive positions in Fig. 2.  The winding here is slightly different, but it amounts to the same thing as far as we are concerned at present.  This is what Mr. Dobrowolsky calls an “elementary” or “simply” rotary current, as used in the Tesla motors.  A similar system, but having three different currents instead of two, is the one used in the Lauffen transmission experiment referred to above.

[Illustration:  FIG. 1.]