Experiment 26

 The Current Balance

 

Objectives

 

To gain further experience with optical levers and to see and measure the forces between current carrying wires.

 

Introduction

 

This is a most sensitive apparatus.  Care must be taken to avoid bending any of the rods or damaging the balance knife edges or their bearing posts.

 

There is a repulsive force between parallel wires carrying currents in opposite directions.  In this experiment, two straight wires are made to carry parallel and identical but opposite currents.  The upper wire is free to move while the lower wire is fixed.  Hence, when the current is on, the upper wire rises and can be returned to its original position by weight of a small mass placed on a pan mounted on the wire.  At that point, the magnetic repulsion is balanced by the weight of the mass in the pan.

 

Procedure

 

Wire the current balance as indicated in the figure.  Be sure the lead wires are perpendicular to the wire segments on the balance.  Balance the upper wire by adjusting the counterpoise behind the mirror until the two wires are separated by about 1 mm (if the wires are not straight or parallel, call the instructor  Adjust the counterpoise below the mirror until the wire frame oscillates with a period of 1 or 2 seconds.

 

 

 

  

Now reposition the frame using the beam life (be gentle).  Set up your light source and adjust and tighten the mirror so the reflected light beam is reproducible on some screen.  Make a final check to see that the balance is positioned on some screen.  Make a final check to see that the balance is positioned correctly and swinging freely.

 

Obtain some small masses and, for as many values as possible between 10 and 200 mg, find the current necessary to balance the weight of these masses.  Measure the lengths of the upper wire through which current flows.  Also, measure the equilibrium distance between centers of the wires by placing a coin on the pan and noting where on the screen the reflected beam arrives.  A little geometry will give you the distance between the wires which is one diameter less than the number you need.

 

Evaluation

Plot F vs I2 for your data.  F is, of course, the weight of each mass and I is the current.  Do a least squares fit to the data and compare your slope with the current value 2 x 10-7L/d where L is the length of the current carrying wire segments and d is the distance between wire centers.