420 



effect thereon was nearly doubled ; and that when many folds 

 of wire were coiled in like manner, the effect was increased 

 by each coil. This is the ordinary form of galvanometer. 



If two needles be connected one above the other, so that 

 one be within the coil and the other above, the ends being 

 reversed, the effect is increased; as both the needles are 

 affected by the current, and the directive influence of terres- 

 trial magnetism is neutralised. From the latter circumstance 

 this arrangement is termed the astatic galvanometer. The 

 astatic galvanometer may be constructed so that the needles 

 move on a horizontal, instead of a vertical, axis, as in the 

 form adopted by Messrs. Cooke and Wheatstone in the elec- 

 tric telegraph now in general use. 



These galvanometers are usually furnished with graduated 

 indices, marked with degrees of a circle, and in many cases 

 are extremely useful for the measurement of weak or mode- 

 rate currents ; but are liable to the material objection that 

 the deflections produced are not in the ratio of the degrees 

 indicated, but in a complicated ratio of tangents of the 

 angles indicated, and of the distance of the needle from the 

 coil further complicated, as the coil does not represent a 

 mathematical line, and by the magnetism of the needle not 

 being strictly permanent. 



The torsion galvanometer is an astatic needle, suspended 

 by fine fibres of silk. This may be made exquisitely sensible, 

 and its indications, as dependent upon the torsion of the 

 fibres, are very consistent ; but it is, unfortunately, not easily 

 applicable when energetic currents are to be measured, and 

 it is an expensive instrument, and liable to be deranged. 



Electro -magnets have been used to estimate energetic 

 currents, by observing the weight which the keeper will 

 sustain ; and electro-magnetic machines have also been em- 

 ployed. These are not very easily applicable, as very slight 

 variations of the distance to which the keeper is allowed to 



