. A text-book of electrical engineering;. plane of the coilis vertical and in the magneticmeridian. When no current is flow-ing, the axis of the needle will he inthe plane of the coil. In the plan ofthe galvanometer (Fig. 48) the coilwill be represented by a straightline parallel to the hues of force ofthe horizontal component h of theearths magnetism. When a currentflows in the coil, each pole m of theneedle will be subjected to a force/ perpendicular to the plane of thecoil; from equation (35) we have r .2tt■^ r ■ The force on each pole due to the Fig. 48 earths field is equal to an
. A text-book of electrical engineering;. plane of the coilis vertical and in the magneticmeridian. When no current is flow-ing, the axis of the needle will he inthe plane of the coil. In the plan ofthe galvanometer (Fig. 48) the coilwill be represented by a straightline parallel to the hues of force ofthe horizontal component h of theearths magnetism. When a currentflows in the coil, each pole m of theneedle will be subjected to a force/ perpendicular to the plane of thecoil; from equation (35) we have r .2tt■^ r ■ The force on each pole due to the Fig. 48 earths field is equal to and acts parallel to the plane of the coil. Under the action of these two forces the needle comes to rest at an angle a to the plane of the coil. The component of the force due to the earth, acting at right angles to the needle, is AB = sin a, while the opposing component of the force due to the current in the coil is . „ . .2-irAC =/.cosa = cos a. Since the needle is in equilibrium, AB = AC, or , . m .r .2Tr cos Cancelling m out from each side of the equation, we see that the reading ofthe galvanometer is quite independent of the strength or size of the mag-netic needle. Solving the above equation for I, we have I = tan a 27r •(37)- Since the number of amperes is 10 times the number of absolute unitswe have •(38). T ^I = —: tan a 2,ir If the horizontal component of the earths field at any place is known,the strength of current can thus be calculated from the values of r and , a standardised tangent galvanometer can be used to deter-mine the horizontal component. 58 Electrical Engineering We wi]l now consider the case in which the pole m lies on the axis A Aof the coil, but at a distance from it (Fig. 49). If the line a joining the poleto the circumference of the coil makes an angle a with the axis, then r sin a Now the element of conductor at B is perpendicular to the plane of thepaper and is therefore at right angles to the li
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