A researcher would like to perform an experiment in a zero magnetic field, which means that the field of the earth must be cancelled. Suppose the experiment is done inside a solenoid of diameter 1.5m , length 4.0m , with a total of 6000 turns of wire. The solenoid is oriented to produce a field that opposes and exactly cancels the field of the earth. What current is needed in the solenoid’s wires?

**Answer 1**

Using Ampere’s Law, the magnetic field produced inside this solenoid is given by

B = uo N I / h

where uo is the vacuum permeability, N is the number of turns in the solenoid and h is the length of the solenoid. Earth’s magnetic field is around 50 microteslas in North America thus the current needed in the solenoid is

I = B h / (uo N) = (50 E-6 ) (4) / ((4 pi E-7)(6000) ) = 0.026 A

I = 26 mA

So you need a current of around 26 mA.

**Answer 2**

An the thickness of the twine? Are those single or overlapped wires? Are there gaps between the wires? Why no longer purely placed a compass interior the tube and strengthen the present till the compass drifts from North? The power of the earth’s magnetic field isn’t the comparable everywhere rmember. Your query quantities to ‘How long is slightly string?’