114 M. F. Kohlrausch on the Determination of the Absolute 



and tenths, zero being where the edges meet. When it is neces- 

 sary to measure the diameter of a wire, a piece is straightened 

 and slid along the angular opening towards the smaller end until 

 it will go no further. Its position, as shown on the graduated 

 straight edge, will then give the diameter in parts of an inch, 

 with sufficient accuracy for practical purposes, to three places of 

 decimals*. For instance, if the distance of the wire from the 

 zero-point be 2"*5, the diameter will be W, 025. 



The thickness of the silk covering is easily measured by halving 

 the difference between the diameter of covered and that of the 

 uncovered wire. 



Although in the Table I have calculated the radii of the wires 

 to six places of decimals, yet this is chiefly to show how small 

 the difference is between the result obtained by solution of the 

 biquadratic equation (2) and that obtained by solution of the 

 simple equation (4). It is, however, of course, practically quite 

 unnecessary to carry the calculation beyond three places. 



It must be remembered that the result obtained is the radius 

 of the wire, and of course must be doubled to obtain the dia- 

 meter. 



XV. Determination of the Absolute Horizontal Intensity of the 

 Earth's Magnetism by measurement of a Current. By F. 

 Kohlrausch f. 



GAUSS'S method of measuring intensity, which together 

 with the determination of declination forms a whole, is 

 characterized by a classical simplicity of instrumental appliances, 

 but requires for great accuracy the highest care of a practised 

 observer. In the Gottingen Nachrichten of May 6, 1868, 1 com- 

 municated a determination of the three terrestrial magnetic ele- 

 ments by means of a new arrangement set up in the Gottingen 

 Observatory, which, while it brought the determination of the 

 inclination, by means of the terrestrial inductor, into the con- 

 nexion between the measurement of the declination and of the 

 intensity, at the same time, by improving the mechanism, essenti- 

 ally diminishes the labour and ensures greater accuracy. This 

 arrangement shall be described in another place. 

 Yet every method of measuring the intensity which is restricted 



* The principle upon which this useful wire-gauge is founded was men- 

 tioned to me some years ago, I think by Mr. Britton of the Electric Tele- 

 graph Company; but I do not know who originated the idea, or MJ?ether 

 any gauges of this form have been made elsewhere than in my work- 

 shops. 



f Translated from Poggendorff's Annalen for September 1869, having 

 been read before the Gottingen Royal Society, February 6, 18G9. 



