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Charge Sphere
France?, second half eighteenth century
The main advantage of a sphere like this is its simple shape. The electric field is easy to calculate, so the experiments may be more easily analyzed mathematically. We also have a charge sphere which is part of a Coulomb balance. Since the purpose of the Coulomb balance is to experimentally measure the repulsion of charges as a function of distance, the mathematical convenience is especially useful. There are other, related devices. To show electrostatic induction, demonstrators would use an elongated charge-holder. A nearby source of charge (such as a main terminal) would induce an opposite charge on the near end, and a like charge on the far end of the charge-holder. There were little hooks mounted below, so threads with pith-balls could be hung to display the local charge density. An illustration of one such experiment, extracted from George Adams' 1792 Essay on Electricity, is shown below, followed by a related charge-holder from our collection.
Elongated Charge-Holder
with Hooks
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A charge sphere is a place to put electrical charge
for experiments. It's just a metal sphere on top of an insulating column,
with a base. Here the insulating column is glass, warmed and coated
with shellac to make it an even better insulator. 
