To the mechanic this instrument will be particularly interesting, because of the exceed ingly ingenious joint, so to speak, which exists at the pivot of the coil. The problem here was to introduce the current into the coil without causing it to pass between moving surfaces, the relations of which might constantly change in conditions of wear, in which ease the resistance to the coil at this point might be of unknown and variable quantity. Leading the current in through the springs, entirely overcomes any difficulty of this kind.
The Weston Alternating Current Voltmeter and Ammeter.—The difficulty of measuring a current which is rapidly alternating or reversing has always been recognized by electricians ; especially when the need was understood of an index which should, despite these quick changes in the current, move steadily to its reading and there stand. Alternating cur rents have hitherto usually been measured indirectly, as by gauging the expansion of a fine wire heated by the current. The Weston instrument consists of a fixed coil held in suitable supports, within which is arranged a movable coil, the axis of the second coil being at right angles to that of the first. The movable coil and the support for the fixed coil (removed) are shown in Fig. 4. The movable coil has combined with it spiral springs arranged in substantially the same way as has already been described in connection with the direct current instrument, and its pivot carries the index needle, which moves over a scale similar to that shown in Fig. 1. The electrical connection of the two coils is such that the current to be measnred passes through both of them, and therefore the field generated around the moving coil reacts upon the field generated around the fixed coil ; and as a consequence the moving coil is caused to move over a distance bearing a relation to the difference of potential between the terminals of the instrument. Of course, changes in the polarity of the current equally affect both coils. If the current reverses in one, it also reverses in the other; so that, despite these reversals, the relation of one field to the other remains the same. Therefore, the movable coil simply traverses over the proper angular distance, depending upon variation in current pressure or current strength. and thus moves steadily up to its scale-marking. and stays there. The great sensi tiveness as well as the simplicity of this instillment is remarkable. By suitable changes in the electri cal connections, and the introduction of resistan ces, the instrument may be adapted either as a volt meter or as au ammeter.
Among the other remarkable electrical measur ing instruments devised by 11r. Weston. is an am meter capable of measuring the strength of the whole current to be ascii by an elect rie-lighting plant. Instruments of this kind have been con structed capable of measuring over• moo ninpTes.
Ile has also devised an entirely novel series of re sistance coils.
Tut: FisKE ELI:11(1CA 1. datNG E•FINDEL—This apparatus involves an entirely novel application of electricity to Ihe measurement of distances at sea. It is the invention of Lieutenant Bradley A. Fiske, of the United States Navy, and its principle will be readily understood from the accompany ing diagram (Fig. 5).
The apparatus proper consists simply of two arcs of conducting material, marked E and F on the diagram, which in reality are merely two lengths of wire supported on the circumfer ence of two circular platforms resting on tripods. Centrally pivoted on each platform is an ordinary spy-glass or telescope, marked C and D in the diagram. Each telescope is provided with an arm or wiper, which sweeps over the wire or arc E or F. always making contact with it. The extremities of the arcs E and F are connected by wires a b c d, which are properly insulated and disposed between-deeks, or in any way so that they will be protected from injury, just as ate ordinary electric lighting oi• other wires. Connected to these wires is the indicating instrument, on the face of which there is a dial marked to indicate yards of range and a pivoted needle or pointer. With the pivots of the telescopes is connected a galvanic battery of any convenient form. This battery, with its conducting wires, may be placed be low in the vessel in some protected position. The electrician will readily see from this diagram that the parts are connected in what is known as a Wheatstone bridge, or electrical balance circuit ; and to him no further description will be necessary to explain the fact that when a balance occurs in the bridge the indicating instrument will show no deflection, and that when the balance is disturbed the deflection of the index will bear a relation to and practically measure the extent of the disturbance. Thus, for example, supposing the two telescopes to be placed in the positions C and P. the wiper on each then making contact with the central portion of each arc, then the resistance which the current will encounter in so much of its path as extends from the center of arc E to the ends thereof, and then through the wires a and c to the indicating instrument, will be equal to the resistance which it will encounter in the remainder of its path, measured front the ten tral portion of arc F through wires b and d to the indicating instrument ; and therefore the index will not be deflected. But if one of the telescopes—C, for example—be moved to the position C., then the travel of the current through the greater part of the arc L' and over the wire c will be over a longer path than when it travels over the less part of the arc E and the wire a; and, consequently, there will be a disturbance in the balance, which will be indicated by the movement of the needle of the index to a new position.