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ID number: BIRRC-P0202 Institution: Research and Cultural Collections Named collection: Historic Physics Collection Artist / Maker: Large sphere: Messrs Storey, Balancing sphere: Messrs Heenan, Two small spheres: Messrs Whitworth Title / Object name: Poynting’s spheres for determination of the gravitational constant Object type: Physics apparatus Place made: Manchester and Birmingham Date made: Large sphere; pre 1879, Balancing Sphere; 1889 Materials: Lead antimony alloy (specific gravity 10.4), small spheres originally gilded Measurements: Large sphere: mass 153.4 kg, diameter 30.5 cm Balancing sphere: 76.5 kg, diameter 24.3 cm Small spheres 21.6 kg, diameter 15.8 cm |
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In 1890 Professor J. H. Poynting performed an experiment to determine the gravitational constant G and hence the mean density of the Earth using a large beam balance to measure the attraction between these massive spheres. The two smaller spheres were suspended from the arms of the balance and the force of attraction of the large sphere on one of them was equivalent to a weight of about 0.3 mg. This large sphere was moved from being under one to being under the other and the middle sized sphere was used as a counterbalance. Poynting’s value for G was 6.6984 x 10^-8 cgs units, compared to the modern value of 6.672. |
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