In Figure 21.109 a bar of length L, width w, and thickness d is positioned a distance h underneath a long straight wire that
carries a large but unknown steady current (w is small compared to h, and h is small compared to the length of the
long straight wire). The material that the bar is made of is known to have n positive mobile charge carriers (“holes”) per
cubic meter, each of charge e, and these are the only mobile charge carriers in the bar. An ammeter measures the small
current I passing through the bar.
(a) Draw connecting wires from the voltmeter to the bar, showing clearly how the ends of the connecting wires must
be positioned on the bar in order to permit determining the amount of current in the long straight wire Iwire. If the
voltmeter reading is positive, does the current in the long straight wire run to the left or to the right? Explain
briefly.
(b) If the voltmeter reading is , what is the current in the long straight wire, ? Express your answer for the
large current only in terms of the known quantities: h, L, w, d, n, , , and known physical
constants such as e.