312 J Mr. Fox's Remarks on the Electro-Magnetism 



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 paper, and in every instance it became affected as soon as the 

 warmth had in any degree penetrated through the bottom of 

 the respective boxes. When other substances, such as slen- 

 der copper wire, paper, &c. were suspended like the needle, 

 they also were agitated by slight changes of temperature; 

 but when the needle was inclosed in an exhausted receiver, 

 it did not appear to be much, if at all, affected by heat. 

 Hence it becomes manifest that the anomalies so often com- 

 plained of in making experiments on the vibrations of the 

 needle, may probably have arisen from the box having been 

 partially affected by changes of temperature, produced, per- 

 haps, by having been held in the hand, or by some slight 

 change of position affecting the temperature of the box ; and 

 when observations are made in the open air, it is evident that 

 the vibrations may be sensibly disturbed by solar heat, cold 

 wind, and other causes. Indeed, I have found from repeated 

 experiments, that when the needle is vibrated in the sun, the 

 arcs become rapidly diminished, and the vibrations conse- 

 quently inci-eased in number ; but in all cases of exposure to 

 warmth, it appears that the vibrations and arcs are very ano- 

 malous, depending, no doubt, on the direction in which the 

 excited aerial currents act on, or strike the needle : — hence 

 the discrepancies which occurred in my experiments above 

 stated. 



It is therefore obviously important that the magnetic needle 

 should be exposed as little as possible to fluctuations of tem- 

 perature, and that it should be contained in a box made of 

 wood, or of some other imperfect conductor of heat ; and for 

 the same reason there would be an advantage in having the 

 glass doubled. The needle itself should not be too light, and 

 the cylindrical form will least expose it to being disturbed by 

 currents of air. It is, however, evident, that no remedy can be 

 so effectual as exhausting the air, which, when it can be con- 

 veniently done, will add much to the value of experiments 

 with the vibrating needle, and render all observations on the 

 compass, in which great accuracy is required, more deserving 

 of confidence*. It might, however, be unnecessary to exhaust 

 the air, if the needle were suspended in a vessel surrounded 

 with water, or nearly so, at a given temperature. Indeed it 

 might sufficiently answer the purpose to have the top and 

 bottom of the vessel or box, furnished with a double metallic 



* It seems that my friend W. S. Harris has for some time past been in 

 the habit of using a vibrating magnetic needle suspended in an exhausted 

 receiver ; and I have very recently seen his apparatus, which appeared to 

 me to be admirably adapted for making experiments on the terrestrial in- 

 tensity. 



