LIEUT. FISKE'S NEW RANGE FINDER.
THE importance of being able to determine the exact position and range of an object to be fired at, whether from a fort or war vessel, is too obvious to need further exemplification, and hence a ready means of determining this point is evidently of the utmost practical importance in gunnery. As our readers may be aware, Lieut. Bradley A. Fiske, U. S. Navy, has devoted considerable attention to this subject, and has designed a variety of forms of these instruments. His most recent work in this branch is a range finder which embodies a decidedly novel application of the Wheatstone bridge as a means of measuring the angles, and by means of which ranges or distances can be read directly from a scale.
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Broadly considered, Lieut. Fiske's latest method consists in determining a fractional portion of a conductor bearing in length a ratio to the angle included between two lines of sight directed upon a distant object, and simultaneously causing a disturbance in an electrical balance, including the conductor in its circuit, proportional to the resistance of the fractional portion, and observing the difference in potential due to the disturbance.
The accompanying diagram, Fig. 2, illustrates the simple and ingenious manner in which this is carried out. We will suppose *AB* to be a base line, and *T* the position of a distant object, the range of which *AT* is to be determined. By trigonometry, in the triangle *ATB*,
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Let *C* and *D* represent two telescopes pivoted at the points *A* and *B* and sweeping over arcs *E* and *F* of conducting material, the arcs having their extremities upon the base line *AB*. Let the telescope *C* be directed upon the point *T1* assuming the position represented by *C'*, in dotted lines. Then obviously, the angle *C'AC* is equal to the angle *ATB* and the portion of the arc *E* included between the positions *C* and *C'* of the telescope, will measure the angle at *ATB*.
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In the foregoing formula, the base line *AB* is known by measurement, and the angle *ABT* may be observed; and if the angle *ABT*, as shown in Fig. 2, a right angle, then the sin *ABT* becomes unity. It remains, therefore, to find the angle *ATB* in order to determine the distance *AT*; so that it becomes necessary to provide a simple and rapid means of at once determining what the angle *ATB* is. To this end, the conducting arcs *E, F*, are connected in the manner of a Wheatstone bridge, the four members of which are shown respectively at *a, b, c, d*. In this bridge is connected a galvanometer in the usual way, and also the battery *h*; the terminals of the battery wire being connected to the telescopes at their pivot points *A, B*, so that the circuit proceeds through the telescopes to the arcs, and then at the arc divides through the wires *b, d*, and at the arc divides through the wires *a, c*.
It will be plain that when the two telescopes *C* and *D* stand at right angles to the base line, and hence parallel to each other, the bridge will balance, and the galvanometer will show no deflection. The lines of sight of the two telescopes then being parallel, the galvanometer will then indicate infinite range; and of course, this will be true no matter where the telescopes may be on their respective arcs, so long as their lines of sight are relatively parallel. But if one telescope be moved out of parallelism with the other, as for example, the telescope *G* moved to the position *C'*, then clearly the bridge will be thrown out of balance, and the galvanometer will be deflected. It will also be clear that the extent of deflection of the galvanometer will depend upon the length of arc included between the two positions of the telescope, *C, C'*, and will be greater as that arc increases; so that with a battery of constant electromotive force, it becomes possible to determine the extent of movement of the telescope *C* by simply observing the indication of the galvanometer.
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