THE INTENSITY OF TERRESTRIAL MAGNETISM. 2G1 



varying changes are entirely overlooked with this instrument, 

 while the new instrument indicates them most satisfactorily. For 

 all other purposes — for instance^ for comparing magnetic Bars 

 with one another in respect to their magnetic strength, — and in 

 connexion W'ith a mvdtiplier, forgalvanometrical and telegraphic;d 

 purposes, — both are alike useful. With respect to the two last 

 applications, the new apparatus has an important advantage, in 

 its being in our power, as above mentioned, to render it as nearly 

 astatic as may be desired. A few instances of the sensibility 

 of the apparatus as a galvanometer may be here noticed. The 

 multiplier surrounding the magnetic bar contains 610 coils cf 

 copper wire covered with silk, and the galvanic current has to pass 

 through a length of wire of more than 6000 feet. This length 

 increases to 13,000 feet if the current at the same time is 

 brought from the physical cabinet. In general, however, other 

 apparatus are brought into connexion Avith the chain, so that in 

 many experiments the whole length of wire amounts to 40,000 

 feet, or nearly tw'o German miles. By far the greatest portion 

 of this w ire is veiy thin ; and this length, in so far as the force 

 of the current is affected by it, is equivalent to a wire about eight 

 German miles in length, of the thickness of the connecting 

 wire betw^een the Astronomical Observatory and the Physical 

 cabinet. Notwithstanding this long chain, even the w-eakest gal- 

 vanic forces give the heavy magnetic bar a deflection not merely 

 perceptible, but sufficing for accurate measurements. This ap- 

 plies to thermo-galvanism, respecting which many philosophers 

 have the eiToneous notion that it cannot pass through a very 

 long chain. With the arrangements at Gottingen, and on the ap- 

 plication of a thermo-galvanic apparatus of peculiar construction, 

 the effect is produced merely by touching the connecting points 

 with the finger. The application to the common electricity 

 of friction gives rise to another interesting observ^ation. It is 

 known that Colladon discovered, by experiments which were 

 at first doubted, but subsequently confirmed by Faraday, that 

 the common electricity of friction, conducted tlirough a multi- 

 plier, deflects the needle in the same manner as a hydro- 

 galvanic current. Faraday was the first to prove that, in a 

 very powerful electrical battery, no more electricity is deve- 

 loped than ver}^ weak hydro-galvanic means of excitation pro- 

 pel in a few seconds through a conducting wire of moderate 

 length. Both the reahty and the small amount of the elcctro- 



VOL. U. PART VI. a 



