Deflection of the Needle
المؤلف:
GEORGE A. HOADLEY
المصدر:
ESSENTIALS OF PHYSICS
الجزء والصفحة:
P-365
2025-12-09
24
Demonstration. - Couple a battery to a straight wire AB (Fig. 1), and hold it above and parallel to a magnetic needle. The needle will turn from its position in the direction indicated in the figure. Place the wire below the needle, and the direction of its deflection is reversed.
Change the direction of the current in the wire, and the needle is deflected as at first. 
This demonstration is important; it shows the principle of one class of measuring instruments.
The relation between the direction of the current in the wire and the deflection of the north end of the needle is what we should expect from the direction of the lines of force around a current : Place the right hand with the palm on the wire and turned toward the needle, and with the fingers extended in the direction in which the current is flowing; then the extended thumb will point in the direction of the deflection of the north end of the needle.
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