Although the thought processes used to answer the previous questions are very helpful as a starting point, it is important to verify these hypotheses with data collected from an experiment. Coulomb did this using a torsion balance. A detailed illustration of the torsion balance is shown in the following illustrations.


Torsion balance:  an instrument designed to measure small electrostatic forces by the twisting of a thin wire

In Coulomb's torsion balance, a horizontal, insulated rod is suspended by a thin, stiff fiber of silver wire. The wire twists when a force is exerted on sphere A, which is shown at the end of the rod. The force on sphere A causes the rod to rotate horizontally. The twisting of the wire can be measured at the suspension head at the top, which indicates the force acting on sphere A.


 




 

A simplified representation of this apparatus focuses on the suspension head at the top and the two charged spheres at the bottom.  his simplified version shows how the variables of distance of separation, r, and charge, q, could be used to measure the electrostatic force .

In this simplified scheme, the ruler can be used to measure the distance from the centre of sphere A to the centre of sphere B. If sphere A and sphere B were given like charges, a force would act on each sphere. This force would push the spheres apart. If sphere A were free to move, then it would be forced to rotate away from sphere B, causing the wire to twist. A larger force acting on sphere A would produce a greater twist in the wire, which could be indicated by the scale on the top.

Although Coulomb had no way of determining the exact amount of charge on an object, he did devise an ingenious method of varying the charge in a controlled way.


Read Determining Relative Charge
To learn more about Coulomb's method for varying charge, read page 528-529 in your textbook. After, answer Self-Check 1


Coulomb used a third sphere, C, which was identical to sphere A and sphere B.  Explain how Coulomb used sphere C to vary the charge on sphere B.

Sphere C was identical to sphere B, but it was neutral. If sphere C was touched by sphere B, then the charge on sphere B would be split evenly between spheres B and C. If sphere B began with an amount of charge that was «math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mfrac»«mn»1«/mn»«mn»2«/mn»«/mfrac»«msub»«mi»q«/mi»«mi»B«/mi»«/msub»«/math»  , after being touched by sphere C, the charge on B would become half of the original value or «math xmlns=¨http://www.w3.org/1998/Math/MathML¨»«mfrac»«mn»1«/mn»«mn»4«/mn»«/mfrac»«msub»«mi»q«/mi»«mi»B«/mi»«/msub»«/math» . This process could be repeated by first grounding sphere C before touching it to sphere B, which would remove half of the remaining charge on sphere B each time.