Experiments designed to illustrate the principles embodied in Ampere’s Law and Faraday’s Law often depend on the knowledge of the number of turns of wire on various types of laboratory coils. The lack of direct measurement of this parameter can be overcome with the use of inexpensive (<$85) digital clamp-on ammeters. The meters themselves illustrate the connection between the current enclosed by a closed path and the line integral of the magnetic intensity H around that contour. In this paper we present laboratory exercises that make essential use of clamp-on ammeters.
REFERENCES
1.
“Microcomputer Interfaced Experiments in Introductory Physics Laboratories,” NSF-ILI Grant No. USE-9050491. 1990–92. Co-PIs S. H. Mellema and C. F. Niederriter.
2.
Copy available by request to [email protected].
3.
C. C.
Jones
, “Faraday’s Law apparatus for the freshman laboratory
,” Am. J. Phys.
55
, 1148
–1150
(1987
).4.
D. C. Henry and C. F. Niederriter, “Enhancing Electromagnetism Experiments with Clamp-On Ammeters,” Summer AAPT Meeting, Boise State University, July 27, 1993.
5.
For examples of the clamp-on ammeter used for measuring electrical appliance energy consumption in a high school physics course see “Encouraging energy saving with a clamp-on ammeter,” by
Lawrence
Ruby
, Phys. Teach.
32
, 510
–511
(1994
).6.
Manufacturers include Fluke Instruments, Radio Shack, Tektronix, B&K Instruments, and others.
7.
P.
Heller
, “Analog demonstrations of Ampere’s law and magnetic flux
,” Am. J. Phys.
60
, 17
–25
(1992
);8.
1995 AAPT Summer Meeting [AAPT Announcer, July 1995, 69, abstract CE6].
9.
The theory of these instruments can profitably be recalled during a following course in electromagnetic theory, as an example of the Reciprocity Theorem. The theory and application of “pure” (i.e., no iron) air-core solenoids (“Rogowski coils”) have been presented by Heller et al. in the previous reference. See also
A. G.
Klein
, “On demonstrations of Amperè’s law
,” Am. J. Phys.
61
, 1045
(1993
).10.
The transfer impedance is operationally defined as the ratio of the induced voltage to the enclosed current.
11.
An identical meter is available under other labels from the following suppliers: Tenma No. 72-645 from MCM Electronics, 650 Congress Park Drive, Centerville, OH 45459-6959 (800-543-4330);
CE Model DM-260T from Alfa Electronics, P. O. Box 8089, Princeton, NJ 08543-8089 (800-526-2532).
12.
Clamp-on ammeters for dc currents are also available, but they use Hall-effect sensors, and are both more expensive and less directly applicable in the experiments cited here. It is, however, nice to have one around for dc current situations.
13.
D. C.
Henry
and S. A.
Danielson
, “Experiments and demonstrations with soldering guns
,” Phys. Teach.
31
, 42
–46
(1993
).14.
R. A. Serway, Physics for Scientists and Engineers, 4th ed. (Saunders, Philadelphia, 1996), Chap. 30, Problem 71, p. 900.
15.
Useful insight into the phase relationships among the applied field, induced emf, eddy current, and consequent magnetic field comes through analogy to the familiar levitating ring demonstration, as analyzed by
Mak
and Young
[ Am. J. Phys.
54
, 808
–811
(1986
)]. We have experimentally verified that the eddy currents in the Helmholtz ring are primarily limited by these self-induction effects, and not the ring’s resistance. We are grateful to one reviewer for calling our attention to this paper, which clarifies the physics of both pieces of equipment.16.
D. C. Henry and J. Kavanaugh, “Audio frequency characteristics of the Sargent-Welch Helmholtz coil,” Summer AAPT Meeting, University of British Columbia, June 29, 1991. We again acknowledge the insightful suggestions of Professor Duane Olson during our study.
17.
Clamp-on probe attachments for connection to a DMM are available from some manufacturers (e.g., Fluke), and these offer sufficient sensitivity and resolution for both applications.
18.
The search coils are in the form of a bobbin made from a 1-cm length of plastic water pipe [hardware store 1 in. type], around which a collar has been cut on a lathe. Between 350 and 550 turns of No. 32 copper magnet wire are wound around this collar, and then the wire ends soldered to twisted-pair hook-up wire terminated in banana plugs. The bobbin and wire connections are epoxied at the end of a wooden dowel rod.
This content is only available via PDF.
© 2001 American Association of Physics Teachers.
2001
American Association of Physics Teachers
AAPT members receive access to the American Journal of Physics and The Physics Teacher as a member benefit. To learn more about this member benefit and becoming an AAPT member, visit the Joining AAPT page.
