Direct "Literal" Demonstration of the Effect of a Displacement Current, страница 4

through the toroidal loop. Finally, a nevv• version could easily be made considerably smaller, but

Notes and Discussions

no harm is done •by ma-king the demonstration easily visible to hundreds of students.

The authors gratefully acknowledge several useful suggestions from Marius Isaila.

E. M. Purcell, Electricity and Magnetism, VOL 2, Berkeley Physics Course (McGraw-Hill, New York, 1965). Section 7.12 and particularly Fig. 7.31 on p. '262 deals specifically with this point.

Simple Laser Test for Uniform Carriage

Motion in Interferometers

L. JOHN GAGLIAIU)I

Ph.ysics Dcparbncnt

Rutgers Universit.y

Camden, Neu Jersey 08102

(R.eeeived 12 July 1973; revised 28 August 1973)

A simple procedure to check for uniformity of advance of the movable element of an interferometer is described in this note. Using this procedure, a record of the uniformity of motion over the entire travel is obtained from which the positions of possible, trouble spots may be easily

determined.

It is most important that the motion of the carriage. on which the movable, element of an interferometer is mounted follow the micrometer advance continuously. Often, the carriageways

are not completely smooth and/or the carriage

gliders become worn. These conditions, separately

or in cornbination, result in an imperceptibly jerky advance of the movable element, with the carriage ' 'hanging up" frequently as it travels. This condition may exist only along a very small portion of the track in an otherwise precise and

well-maintained instrument. The results of even a very limited segment being in this condition can be quite serious in terms of the. resulting experimental errors. As a practical matter, it is wise to consider an interferometer suspect until proven otherwise.

The following procedure may be used to check the entire travel for uniform carriage. motion with micrometer advance. To make the discussion spe-

FIG. 1. Schematic diagram of experirnental arrangement for test of uniform motion of the movable element, in a Michelson interferometer.

cific, the procedure will be explained for a Michelson interferometer. The basic arrangement is shown in Fig. 1; similar arrangements can be

used for several other types of interferonwters. A      

low power, cw laser is directed through a diverging lens and into the Michelson interferometer m. The fringes arc projected onto an inexpensive photodiode p mounted behind a screen s and

variable slit st. The screen, which is used for visual alignment of the fringes, permits exposure of the detector through a fixed slit with a width

approximately equal to the maximum aperture of the variable slit. The detector system consists of a Keithley 610C electrometer e and a strip-chart recorder r. The recorder drive is attached through a gear box (not shown) to the carriage advance, of the interferometer. In this fashion, a plot of the variation in intensity as the fringes sweep

AJP Vol.u,mc 42 / 249