HC-1 Helmholtz Coil Assemblies - Magnetic Shield - #2

Introduction

Operation

The Helmholtz Coil Assembly provides a uniform magnetic field for measurement of magnetic shield per­formance under controlled conditions. It can also be used to determine the sensitivity of equipment to out­side interference in the DC to 10 kHz range. For both sizes, the ratio of the coil diameter to turns is chosen to maintain the convenient ratio:

Hp = I

Hp - field strength (peak Oersteds) I - AC current (rms amperes)

When an rms-indicating ammeter is used, the field in­side the coil is numerically equal to the current input.

Our standard models have 14 gauge windings, which allow intermittent currents to 10 amperes. Typical test currents are 0.5 and 2.0 amperes, permitting con­tinuous operation under normal testing requirements.

A variable autotransformer or other AC power supply capable of 10 ampere output is required, as well as an accurate current-measuring instrument. A standard power cord is provided for connection to the supply source.

Testing of magnetic shield designs in use requires compensation for fluctuations of interfering field strength with time and distance. The Helmholtz Coil Assembly provides a uniform magnetic field in which magnetic shield designs can be evaluated. Helmholtz coils are also used to provide uniform, precise fields to determine the sensitivity of an item to outside interference.

Field Measurement

Magnetic Shield Corporation's AC Magnetic Field Evaluator Probes provide convenient and economical means for measuring fields inside the Helmholtz Coil Assembly. Depending on shield shape, model EP-102A (Axial) or EP-102T (Transverse) may be chosen. The measurement plane of the probe (indicated by the plane of the yellow stripe) must be parallel to the planes of the Helmholtz coils for proper readings. (See photo). Measurement of the output of the probe with and without the shield in place allows calculation of the shielding attenuation ratio.

EP-102T, Transverse Probe enters shield for test.

Construction

Our Helmholtz Coil Assemblies are designed in accord with applicable portions of ASTM standard A698, to be compatible with typical industry practice. No metals other than the copper field coils are used in construc­tion, exceeding the requirements of the standard.

A sturdy oak base supports the coils and contains the electrical wire harness. The solid top provides a con­venient support surface for magnetic shields during testing. Multiple support points for each coil provide rigid alignment without the need for inter-coil braces that would restrict test access.

Disassembly for storage conserves valuable laboratory space. Removal of the top of the base allows access to the wiring harness and removal of the coils. Coil con­nector plugs are polarized to prevent incorrect reas­sembly. No tools are required for disassembly or reassembly.

Specification

Selection of a Helmholtz Coil Assembly begins with coil diameter. ASTM standards specify coil diameter at least as large as the greater of four times the shield diameter or three times the shield length. These ratios are for a typical cylindrical test specimen, tested with its axis parallel to the planes of the coils. Forty-eight inch diameter coils are used in many labs, while the smaller twenty-four inch model consumes less space if only small items will be tested. Consult Magnetic Shield Cor­poration for information on other sizes.

SIZE 24 in. 48 in.

MODEL HCA-224 HCA-248

PRICE $2,150.00 $4,925.00

Perfection Mica Co. 740 N. Thomas Drive Bensenville, IL 60106 U.S.A.

Fax 630-766-2813 Phone 630-766-7800

CATALOG HC-1C

© 2009 MAGNETIC SHIELD CORP., PERFECTION MICA CO.

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