to the determination of the Magnetic Inclination. 155 



to give the signs, proceed from a ffalvanic battery; in the other they 

 are induced in the electric conductor by means of magnetic force. 

 Weber has constructed a telegraph of the latter kind, where 

 the earth's magnetic force induces electric currents strong enough 

 to be used for giving signs. For such induction, either the 

 horizontal oi- the vertical component of the earth's magnetism 

 may be used. Let us suppose the telegraph to be a so-called 

 needle-telegraph (such as are used in England), in which the 

 current gives the sign by passing through a helix and de- 

 flecting a needle suspended in the same from the magnetic 

 meridian. It is evident that, by an exact comparison of the 

 deflections, we may determine the ratio of the intensities of the 

 electric currents respectively induced by the vertical and hori- 

 zontal component of the earth's magnetic force; this ratio is 

 the tangent of the required inclination. In constructing such a 

 telegraph, therefore, it is necessary to obtain deflections of the 

 needle which are capable of accurate measurement, and at the 

 same time to exckide all disturbing influences. For the former 

 purpose it is clear, that, in place of the common magnetic needle 

 of the galvanometer, a magnetometer must be used. To exclude 

 all disturbing influences it is necessary, — 1, that no commutation 

 of the conducting wires be employed in order to increase the 

 deflection ; 2, that the amount of deflection be independent 

 of the velocity of the inductor's motion. For, in the first place, 

 the circuit must remain unchanged if the observed deflections are 

 to furnish an exact comparison of the forces by which they were 

 produced ; and in the second place, these forces must not be 

 subject to variation, as would be the case if they were dependent 

 upon the velocity of the inductor's motion. It will be shown 

 that both conditions are fulfilled when the inductor's whole 

 motion is limited to an induction stroke, capable of momentary 

 execution, and strong enough to produce in the magnetometer 

 a deflection capable of the finest measurement. Such a simple 

 induction stroke consists in a semi-rotation of the inductor ; no 

 commutation in the connexion of the inductor wire with that of 

 the galvanometer is here necessary, because during such a semi- 

 rotation no change in the direction of the induced current takes 

 place. 



We proceed next, by help of the figures in Plate II., to give 

 a description of thi.s new instrument, to which Weber has given 

 the name induction-magnetometer. 



A transverse section of the inductor is shown at A, fig 1. The 

 circumference of the cylinder on which the wire is coiled is 718*3 

 milliins., its diameter is aa = a'a', and its breadth aa' = 120'05 

 milliuis. The copper wire around this cylinder is 51'2296 

 miilims. long, and has a mean sectional area of 4" 145 square 



M2 



