ASTM_A_712_-_14.pdf

Designation:A71214Standard Test Method forElectrical Resistivity of Soft Magnetic Alloys1This standard is issued under the fixed designation A712;the number immediately following the designation indicates the year oforiginal adoption or,in the case of revision,the year of last revision.A number in parentheses indicates the year of last reapproval.Asuperscript epsilon()indicates an editorial change since the last revision or reapproval.1.Scope1.1 This test method covers the measurement of electricalresistivity of strip or bar specimens of soft magnetic alloys.1.2 The values stated in SI units are to be regarded asstandard.No other units of measurement are included in thisstandard.1.3 This standard does not purport to address all of thesafety concerns,if any,associated with its use.It is theresponsibility of the user of this standard to establish appro-priate safety and health practices and determine the applica-bility of regulatory limitations prior to use.2.Referenced Documents2.1 ASTM Standards:2A34/A34M Practice for Sampling and Procurement Testingof Magnetic Materials3.Summary of Test Method3.1 The electrical resistance of a 0.25-m long(minimum)test specimen is measured with a Kelvin-type resistance bridgeor a digital ohmmeter or the potentiometer-ammeter method.The resistivity is then calculated from the resistance measure-ment and the dimensions of the specimen and is known as theelectrical resistivity of the material.This value is equal to theresistance between opposite faces of a cube of unit dimensions.4.Significance and Use4.1 This test method is suitable for the measurement of theelectrical resistivity of specimens of soft magnetic materials.4.2 The reproducibility and repeatability of this test methodare such that it is suitable for design,specification acceptance,service evaluation,quality assurance,and research and devel-opment.5.Apparatus5.1 Kelvin-type resistance bridge or a digital ohmmeter(Note 1)or a dc potentiometer and dc ammeter providingresistance measurements to an accuracy within 0.5%of theaccepted true value.NOTE1A digital multimeter with a four-wire resistance measuringcapability can be substituted for a digital ohmmeter.6.Sampling6.1 Samples shall be representative of the material in thephysical condition as shipped or agreed upon by the producerand the user.7.Test Specimen7.1 The test specimen shall be a straight strip or bar or wireof substantially uniform cross-sectional area.7.2 It shall have a minimum length of 0.25 m.7.2.1 Bars and wires having circular,rectangular,or othersections shall be used in the sectional dimensions as produced,unless they are so large as to require cutting a representativesample of suitable dimensions.7.3 It shall be free of obvious surface defects.7.4 The surface shall be cleaned by wiping with a cloth.Oiland grease,if present on the surface,shall be removed with asuitable solvent.Normal surface oxide or core plating need notbe removed except in areas in which it is necessary to makesatisfactory electrical contact.8.Procedure8.1 Measure the electrical resistance of the test specimenusing a Kelvin-type resistance bridge or a digital ohmmeter orpotentiometer-ammeter system having separate current andpotential leads.8.2 For strip specimens,measure the length of the testspecimen to within 60.1%and weigh the specimen using abalance or scale capable of determining the mass within60.1%.Determine the average cross-sectional area using Eq1.8.3 For bar or wire specimens,determine the cross-sectionalarea by direct measurement using calipers or micrometerscapable of measuring within 60.1%.1This test method is under the jurisdiction of ASTM Committee A06 onMagnetic Properties and is the direct responsibility of Subcommittee A06.01 on TestMethods.Current edition approved May 1,2014.Published May 2014.Originallyapproved in 1975.Last previous edition approved in 2013 as A71207(2013).DOI:10.1520/A0712-14.2For referenced ASTM standards,visit the ASTM website,www.astm.org,orcontact ASTM Customer Service at serviceastm.org.For Annual Book of ASTMStandards volume information,refer to the standards Document Summary page onthe ASTM website.Copyright ASTM International,100 Barr Harbor Drive,PO Box C700,West Conshohocken,PA 19428-2959.United States1 8.4 The distance between each potential lead contact and thecorresponding current lead contact shall be at least twice thewidth of the test specimen with the two potential contacts lyingbetween the current contacts.The distance between the poten-tial contacts shall be not less than 0.12 m and shall be knownto within 60.1%.8.5 The dimension of each potential contact in the directionof the length of the specimen shall be not more than 0.5%ofthe distance between potential contacts.8.6 The contacts to the specimen shall be located centrallywith respect to the specimens width dimension,and thecurrent contacts shall cover more than 80%of the width.Areliable contact shall be made with the specimen by both thecurrent and potential leads