US2730641A

US2730641A - Magnetic compensator

Info

Publication number: US2730641A
Application number: US360978A
Authority: US
Inventors: Thomas A Mclauchlan
Original assignee: United States Steel Corp
Current assignee: United States Steel Corp
Priority date: 1953-06-11
Filing date: 1953-06-11
Publication date: 1956-01-10
Anticipated expiration: 1973-01-10

Description

Jan. lO, 1956 T. A. MOLAUCHLAN 2,730f64l MAGNETIC COMPENSATOR Filed June 1l, 1953 ff/Mm-X [noem:

Unite MAGNETIC COMPENSATUR Application .lune 11, 1953, Serial No. 360,978

2 Claims. (Cl. 313-75) This invention relates to an improved compensator for changeable local magnetic fields.

My compensator is especially suited for electron microscopes, but it is apparent that it can be used with other mechanisms which are similarly disturbed by changeable local elds. In the example of an electron microscope, an electron beam from a suitable source passes through an evacuated specimen chamber and impinges on a specimen at an angle of incidence of approximately 90. Any magnetic field that acts on this beam tends to deflect it away from the specimen. Such microscopes commonly are equipped with means which compensate for constant magnetic fields that act on this beam, such as that of the earth. However, those with which l am familiar lack effective means to compensate for changeable fields, and it often has been necessary to resort to complete magnetic shielding to overcome the effects of such fields.

An object of the present invention is to provide an improved and simplified compensator which effectively overcomes the effects of local changeable magnetic fields and which eliminates the need for elaborate shielding of the mechanism to which it is applied.

A further object is to provide an improved compensator which includes opposed hook-shaped arms of magnetic material arranged to have induced therein a magnetic field proportional to and opposite a disturbing field, whereby the latter is effectively compensated.

In accomplishing these and other objects of the invention, I have provided improved details of structure, a preferred form of which is shown in the accompanying drawing, in which:

Figure 1 is a top plan view, partly in section, of a compensator which embodies features of my invention; and

Figure 2 is a vertical sectional View on line lI-II of Figure l.

As an example of suitable environmental structure for the compensator of my invention, the drawing shows a portion of a cylindrical specimen chamber of an electron microscope. This chamber is formed of nonmagnetic material and is evacuated after insertion of a specimen indicated at X. An electron beam a--b from a source Y beyond the opposite end of the chamber passes therethrough along the central axis and impinges on the specimen. Any local changeable field, such as that indi` cated in Figure l, tends to defiect this beam away from the specimen and thus prevents proper observation thereof.

The compensator of my invention comprises a pair of opposed hook-

shaped arms

12 and 12a which are of a soft magnetic material and are of like construction. Each arm has a screw-threaded shank 13 and a reversely bent tip 14, which are axially aligned. The two arms are held together as a rigid assembly by pairs of clamping bars 15 of a conductive nonmagnetic material, such as aluminum or brass, which does not acquire an electrostatic charge when exposed to an electron beam. In their clamped position the Shanks and

tips

13 and 14 of both arms are all in alignment, and the two tips 14 are separated by a gap 16.

Zfiil iatented Jan. 10, 1956 The assembly of the two

arms

12 and 12a and the clamping bars 15 is mounted inside the chamber 10 with the common axis of the Shanks and tips extending across the diameter of the chamber parallel to the direction of the external changeable field. The mounting means illustrated includes spacers 17 which fit against the inside wall of the chamber and nuts 18 which are threadedly engaged with the Shanks and abut said spacers. The spacers and nuts preferably are formed of the same material as the clamping bars 15. The electron beam a-b passes through the gap 16 between the two tips 14.

A pair of bushings 19 and 19a are xed to the exterior of the chamber 10, for example by soldering or adhesively. The bushing 19 is attached opposite the shank of the arm 12 and the bushing 19a opposite that of the arm 12a. Plugs 2) and 20a are threadedly engaged with the bushings 19 and 19a respectively for adjustment toward and away from the Shanks of the arms. These plugs preferably are of similar material to the arms and are axially aligned with the Shanks and tips thereof.

The changeable local field always induces a proportional eld in the

arms

12 and 12a. The induced field is of course much more intense than the external eld, but it acts on the electron beam only in the relatively small region of the gap 16. The exact relation between the intensities of the two fields can be adjusted by moving the plugs 2i)l and 20a closer to or farther from the ends of the

arms

12 and 12a. The reverse bends in these arms causes the field induced therein to act in a direction opposite that of the external field and consequently compensates for the latter.

From the foregoing description it s seen that the present invention affords a compensator of very simple construction that effectively overcomes disturbances caused by local changeable magnetic fields. It eliminates any need for the more expensive and cumbersome magnetic shielding.

While I have shown and described only a single embodiment of the invention, it is apparent that modifications may arise. Therefore I do not wish to be limited to the disclosure set forth but only by the scope of the appended claims.

I claim:

l. 'I'he combination, with a device which includes a cylindrical chamber and means for passing an electron beam through said chamber along its central axis, of a compensator for changeable local magnetic fields of a known direction comprising a pair of hook-shaped arms of soft magnetic material each having a shank and a reversely bent tip aligned with its shank, non-magnetic and conductive means clamping said arms together with their tips separated to form a gap and the Shanks and tips of the two arms all in alignment, and means mounting said arms inside said chamber with said Shanks and tips lying along a diameter of the chamber in the direction of the changeable local field and with the central axis of the chamber passing through said gap.

2. A combination as defined in claim 1 including mounting means on the exterior of said chamber opposite the extremities of said Shanks, and plugs of soft magnetic material adjustably supported in said exterior mounting means in alignment with said Shanks and tips for Varying the intensity of magnetic field induced in said arms.

References Cited inthe file of this patent UNITED STATES PATENTS 2,153,643 Roosenstein et al Apr. l1, 1939 2,425,125 Richards Aug. 5, 1947 2,637,000 Page Apr. 28, 1953 2,651,000 Linder Sept. 1, 1953