PASCO ES-9070, Instruction Manual And Experiment Manual

Coulomb Balance

012-03760E

4

IMPORTANT: 

If you live in an area where

humidity is always high, and if you have no facilities

for controlling humidity, the experiment will be

difficult, if not impossible, to perform. Static

charges are very hard to maintain in a humid

atmosphere because of surface conductivity.

Experiments with the Coulomb Balance are

straightforward and quite accurate, yet, as with any

quantitative electrostatic experiment, frustration lurks just

around the corner. A charged shirt sleeve, an open

window, an excessively humid day—any of these and

more can affect your experiment. However, if you

carefully follow the tips listed below, you’ve got a good

start toward a successful experiment.

• Perform the experiment during the time of year when

humidity is lowest.

• Perform the experiment in a draft-free room.
• The table on which you set up the experiment should

be made of an insulating material—wood, masonite,

plastic, etc. If a metal table is used, image charges

will arise in the table that will significantly affect the

results. (This is also true for insulating materials, but

the effect is significantly reduced.)

• Position the torsion balance at least two feet away

from walls or other objects which could be charged

or have a charge induced on them.

• When performing experiments, stand directly behind

the balance and at a maximum comfortable distance

from it. This will minimize the effects of static

charges that may collect on clothing.

• Avoid wearing synthetic fabrics, because they tend to

acquire large static charges. Short sleeve cotton

clothes are best, and a grounding wire connected to

the experimenter is helpful.

• Use a stable, regulated kilovolt power supply to

charge the spheres. This will help ensure a constant

charge throughout an experiment.

• When charging the spheres, turn the power supply

on, charge the spheres, then immediately turn the

supply off. The high voltage at the terminals of the

supply can cause leakage currents which will affect

the torsion balance. A supply with a momentary

“power on” button is ideal.

• When charging the spheres, hold the charging probe

near the end of the handle, so your hand is as far

from the sphere as possible. If your hand is too close

to the sphere, it will have a capacitive effect,

increasing the charge on the sphere for a given

voltage. This effect should be minimized so the

charge on the spheres can be accurately reproduced

when recharging during the experiment.

• If you are using a PASCO Electrometer (Model

ES-9035 or ES9054A) to measure the charge on the

spheres, connect the voltage output to a digital

multimeter so that values can be measured more

accurately. It is also useful to calibrate the

electrometer. This is done by applying a calibrating

voltage to the input and measuring the electrometer

output on the digital multimeter. Your measured

values can then be adjusted as necessary.

• Surface contamination on the rods that support the

charged spheres can cause charge leakage. To

prevent this, avoid handling these parts as much as

possible and occasionally wipe them with alcohol to

remove contamination.

• There will always be some charge leakage. Perform

measurements as quickly as possible after charging,

to minimize the leakage effects.

• Recharge the spheres before each measurement.

Tips for Accurate Results