N 24,850                                                                  A.D. 1904

It is well known that alternating electric currents can only bedetected and measured by the use of some instrument, the indications of which do not depend upon the direction of the current. Hence an instrument of which the indication is dependent upon the heating of a wire by the current commonly known as a hot wire instrument is suitable for the above purpose. Also one depending upon- the attraction or repulsion of electric current, commonly known as an electro- dynamometer, because in both these cases the indication or reading of theinstru- ment is tho same even if the current direction is reversed.

On the other hand, an ordinary galvanometer of the movable needle or movable , coil type cannot by itself be used for detecting or measuring alternating currents because it only gives an indication with a unidirectional current and is therefore unaffected by an alternating current.

The object of my invention is to provide a means by which an ordinary galvanometer can be used to detect and measure alternating electric currents and cspecially high frequency alternating currents commonly known as electricoscüla- tions.

The means I employ for this purpose consists in the insertion in the circuit of the alternating current of an appliance which permits only the passage of electric current in one direction and constitutes therefore an electrical valvc.

I construct it as follows: -In ,a glass bulb, I seal two or more carbon filaments such as a.re used for the manufacture of electric lamps. These filaments eachhave their own separate terminals. One or more of these carbon filaments may be replaced by loops of platinum wire, provided at least one carbon filamentis. used. A high vacuum must be made in the bulb. Two such bulbs are employed and for the sake of simplicity, I may describe the mode of use when a double carbon filament. in eachbulb is used. The two bulbs arc each associated with a small insulated primary or secondary battery of sufficient voltage to bring one of the filaments to bright incandescence of greater intrinsic brilliancy than if used -as an incandescent lamp.

The bulbs are connected in parallel with each other and inserted in the alternating current circuit, but so that an incandescent filament and a dark or cold filament in each. bulb form the electrode or connection for the current to be measured to enter and leave the bulb.

. In series with one of the bulbs is placed anordinary galvanometer. In the two branch circuitsformed of the two bulbs, one in series with a galvanometer the local insulated batteries arc arranged to ignite carbon filaments at opposite ends.

That is to say, the carbon filadioiif-nearest to one common terminal is the ignited filamerit in one bulb and that farthest from the same common terminal in the other bulb.

These bulbs have the property that inside the bulb, negate electricity can move from the hot to the cold carbon filament even under a very low electromotive force but it cannot move in the opposite direction. Hence, owing to the arrangement of the bulbs, the alternating current arriving at the common terminal splits and all the positive alternations pass through one bulb and all the negative through the other bulb.The galvanometer is thereforea-ffected solely by currents flowing in one direction. The bulbs serve the purpose of separating out the two constituents of the alternating current.

In making the connections of the bulb to the alternating current circuit it is essential to join one end of the galvanometer wire to that terminal of the hot carbon filament to which the negative end of the local or heating battery isapplied.

I, JOHNAMBROSE FLEMING, of University College, Gower Street, in the County of London, Doctor of Science, do hereby declare the nature of this invention and in what manner the same is to be performed to bo particularly described and ascertained in and by the following statement:---

This invention relates to certain new and useful devices for converting alternating electric currents and especially high frequency alternating electric currents or electric oscillations into continuous olectric currents for the purpose of making them detectable by, and measurable with, ordinary direct currentinstru- ments such as a "mirrorgalvunometer"of the usual type or any ordinary direct current ammeter. .. ~

Such instruments as the latter are not affected by alternating electric currents either of high or low frequency which can only be measured and detected by instruments called alternating current instruments of special design. It is, however, of great practical importance to be able to detect feeble electric oscillations, such as are employed inI3ertzian wave telegraphy by an ordinary movable coil or movable needle mirror galvanometer. This can be done if the alternating current can be ".rectified ", that is either suppressing all the constituent electric currents in one direction and preserving the others, or. else by changing thedirec- tion of one of the sets of currents which compose the alternating current so that the whole movement of electricity.is in one direction. Many means have been devised and are in use for rectifying low frequency alternating currents, such as are used in electric lighting. There are well known forms of mechanical rectifier', also there is a well known form of electro-chemical rectifier depending on the fact that when a plate of carbon and aluminium is placed in any electrolyte which yields oxygen on electrolysis, an electric current can only pass through this cell in one direction if below a certain voltage.

I have discovered that if two conductors are enclosed in a vessel in which a good vacuum is made, one being heated to a high temperature, the space between the hot and cold conductors possesses a unilateral electric conductivity, and negative electricity can pass from the hot conductor to the cold conductor but not in the reverse direction.

As the hot conductor should beheated to a very high temperature say near to the melting point of platinum(1700 ' C. ), it should be of carbon preferably in the form of a filament such as is used in any ordinary incandescent electric lamp. The cold conductor may be of many materials, but I prefer a bright metal such:is platinum or aluminium or else carbon.

The two conductors arc enclosedin' a glass bulb similar to that of an incan- descent lamp, and I generally heat the carbon filament to a high state of incan= descence by atcontinuous electric current, the electrical connection to the filament and the cold conductor being made by platinum wires, sealed air tight through the glass.

In Figure 1 a is a glass bulb, andb is a carbon filament like the carbon fila-inent of an incandescent lamp, suitable say for taking a current of G to 8 volts and 2 to 4 amperes. c is a cylinder of aluminium open at the top and bottom which surrounds but does not touch the filament. The cylinderc is suspended and steadied by platinum wiresd, and the ends of the filament6 are connected to platinum wires connected to the leads e and f. The platinum wires are sealed through the glass in the ordinary manner.

As a very high vacuum should be obtained in the bulba and as a considerable quantity of air is occluded in the conductors these should be heated when the bulb is being exhausted. The filament b can be conveniently heated by passing a current through it whilst the cylinder c can be heated by surrounding the bulb a with a resistance coil through which a current is passed the whole being enclosed in a box lined with asbestos or the like. When as hereinafter described the cylinder c is replaced by any form of conductor which can be heated by passing a current through it this method is usually more convenient than that just described.

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