Magnetic Storms

Abstract

IT is surprising that an accomplished telegraphist like Mr. Mance (NATURE, vol. xix. p. 409) should not see the necessity and advantage of expressing earth-currents in webers. It is precisely because every one can, if he likes, appreciate the magnitude of an earth-current so expressed, and no one but himself can do so if Mr. Mance's plan were adopted, that I advocate the weber, or rather, its more convenient sub-multiple, the milliweber. A milliweber is the current produced by one Daniell's cell (strictly one volt), through 1,000 ohms. Currents can be reduced to this unit from any galvanometer. The tangent galvanometer is, perhaps the simplest to use–it is that which we employ in England. Supposing for simplicity that your constant, viz., one Daniell cell through 1,000 ohms (including cell and galvanometer) gives 45°, then the tangent of any other reading will give you the current in milliwebers. Then, knowing the resistance of your circuit and its geographical position, you have all the data necessary to determine the elements of earth-currents.