Imágenes de páginas
PDF
EPUB

Proceedings of the American Academy of Arts and Sciences.

VOL. XLII. No. 9.- JULY, 1906.

CONTRIBUTIONS FROM THE JEFFERSON PHYSICAL LABORATORY,

HARVARD COLLEGE.

A SIMPLE DEVICE FOR MEASURING THE
DEFLECTIONS OF A MIRROR

GALVANOMETER.

BY B. OSGOOD PEIRCE.

CONTRIBUTIONS FROM THE JEFFERSON PHYSICAL LABORATORY,

HARVARD COLLEGE.

A SIMPLE DEVICE FOR MEASURING THE DEFLECTIONS OF A MIRROR GALVANOMETER.

BY B. OSGOOD PEIRCE.

Presented April 11, 1906. Received May 25, 1906.

FOR a good many years we have used in the Jefferson Laboratory, to determine the deflections of mirror galvanometers and other similar instruments, a simple form of tubeless telescope, which, at first intended for students only, has proved in its way very convenient, and has been adopted for some standard fixed instruments, because it is not easily put out of adjustment after it has once been properly set up, and because the scale image is so large and clear that a prolonged use of the device does not involve the eye strain which most reading telescopes would cause.

B

S

A

H

FIGURE A.

In front of the plane mirror (M) of the galvanometer, instead of the usual cover glass, is placed (Figure A) a convex lens (A) of focal length equal to the desired scale distance. This lens need not be achromatic; a spectacle lens is quite good enough. At a distance in front of A equal to its focal length is placed a horizontal scale (S) mounted on a thin strip of wood at least twice as wide as the scale itself. Through the middle of this strip, above the scale, is bored a round hole (H) rather more than 20 millimeters in diameter; just behind the scale is stretched a fine vertical wire or silk fibre (W) to serve as a cross-hair. Behind H and at a distance suited to its focal

length, and to the eye of the observer, is placed another spectacle lens (B) to serve as an eyepiece. The centre of H should lie approximately in the common axis of A and B. A peephole (P) on this same line is placed at such a distance behind B, which may conveniently have a focal length of 12 or 15 centimeters that B's aperture shall appear wholly filled with a large, clear, uncolored image of the scale, with the wire W running vertically across it. The proper distance between A and S is determined by making the parallax of W across the lines of the scale, when the eye is moved sidewise across the peephole, to disappear when this distance has once been found, the scale is firmly clamped in position. The distances between S and B and between B and P may be adjusted by every person to please himself, though it is usually possible to fasten P securely in a suitable position and to move B only to fit the eyes of different observers. It is important to notice that if the distance between the eye-lens B and the peephole P is not properly chosen, only a small round portion of the image of the scale will be seen, not nearly large enough to fill the aperture of B, and this will appear to move about in a distressing manner, when the eye at the peephole is moved slightly. When the simple adjustments have been properly made, however, the eye need not be pressed close to the peephole, for the deflections of the galvanometer can be accurately and easily read whether the eye is one inch or four or five inches behind P. It is not necessary to close the unused eye, for the field should be extremely bright and clear; the observer is, of course, looking at a life-size image of the scale at a distance PH from the peephole, and it is easy to get any desirable magnification of this image by a proper choice of the eye-lens. Ordinary astigmatism on the part of the observer's eye can be partially corrected by a suitable spectacle lens at B. One slight drawback to the use of this reading device, where space is limited, is the fact that the eye will generally need to be 25 or 30 centimeters behind the scale. When it is desirable to do so, the eye-lens, B, may be mounted in a tube which slips over the tube which carries the peephole.

Figure 1 shows one form of the arrangement just described, mounted, for students' use, with a mirror amperemeter on a wooden frame. Several adjustments are here provided for, so that different instruments may be used on this frame. Figure 2 shows the scale and eyepiece as mounted on a brass rod, by Mr. J. Coulson, and Figure 3 represents a convenient way of using the device with a wall galvanometer.

THE JEFFERSON PHYSICAL LABORATORY,

HARVARD UNIVERSITY.

[merged small][graphic][merged small][merged small][graphic][merged small][merged small]
« AnteriorContinuar »