Publication: The Electrical Engineer
New York, NY, United States
LIEUT. FISKE'S POSITION FINDER.
IN a recent issue¹ we described the valuable range finder of Lieut. Bradley A. Fiske, by which a gunner is enabled to know accurately the distance of the object which he is firing at. This instrument can be applied in many situations on land and on board ship, butpresupposes, of course, that the gunner is able to sight directly upon his target. In modern fortifications, however, the tendency to the employment of what are known as disappearing carriages is becoming stronger every day and hundreds of such carriages are already employed in sea coast forts abroad.
In these forts the guns are loaded below the level of the parapet and when ready to fire are raised into firing position, usually by a hydraulic ram. This method of operation evidently protects those charged with the loading of the gun while the operation of loading is in progress; but it is evident, that by the means heretofore at hand, it has been impossible to sight the gun while depressed behind the parapet. True, the range could be determined beforehand by means of the range finder so as to permit the gun to be given the correct elevation before raising to fire, but this gives no indication of the true direction of the object to the concealed gunners, and hence deprives them of an essential factor in the pointing of the gun.
What was wanted, therefore, was some means by which the gunner could obtain both the range and direction of the object, with regard to the gun as a central point, in other words, their relative position. It is this which Lieut. Fiske has now accomplished, and in virtue of which the gunner is enabled to locate on a chart, drawn to scale, the exact position of a distant object which he has no means of seeing, but which is being sighted on by two observers placed at any distance, well protected from the fire of the enemy and unobscured by smoke.
The new position finder may be said to be a simplification and amplification of the range finder. It embodies, first, a telescope moving over an arc of conducting material and which is directed upon the distant object; second, a pivoted pointer moving over a like arc of conducting material in a Wheatstone bridge circuit with the first-named arc; the arrangement being such that when the alidade arm attached to the telescope on the first arc, and the pointer on the second arc stand at the same angle, the circuit is balanced. The pointer arm moves over a chart representing the area which includes the position of the distant object on a reduced scale. On this chart there is a simple pivoted arm which can be trained directly upon the object; or the arm may be mechanically controlled by a telescope directed upon the object so that it will make with the other arm an angle equal to that made by the lines of sight drawn from the two telescopes to the object. The position of the object is then shown by the intersection of the electrically directed pointer and the mechanically directed arm upon the chart.
In the accompanying illustrations, Fig. 1 is a diagram showing the general arrangement, and Fig. 4 is a similar diagram showing the chart D in different position, AB represents, for example, the parapet of a fortification. The distant object is supposed to be located at C; and it is the position of this object which is to be determined upon a chart D, on which the fortification line A'B' appears on a reduced scale, E is an arc of conducting material and F a telescope or arm pivoted at one end at F' with its free extremity moving over, and making contact with, the arc E. G is an arc similar in all respects to the arc E and located in proximity to the chart D. H is an arm pivoted at H' and having its free end sweeping over, and making contact with, the arc G and carrying a pointer I. a, b, c, d, are members of a Wheatstone bridge connecting the arcs E and G. E is a loop including the battery, and f the loop including the galvanometer g.