Electro-physiology and electro-therapeutics : showing the best methods for the medical uses of electricity / By Alfred C. Garratt.

it, is placed either above or below a magnetic needle which is freely suspended and parallel to its direction, the needle is im- mediately deviated, and all the more considerable as the gal- vanic current is more powerful. It tends to place itself trans- verse and perpendicular to the conductor wire of the battery — a position it nearly attains, provided it is near, and the battery current is sufficiently strong. In fact, the extent of the devia- tion of the needle is directly proportional to the power of the battery, and inversely proportional to the distance between the needle and the wire. But if it is a common magnetic needle, it will not be deflected so far as to assume a position exactly at right angles with the conducting wire, on account of the influ- ence of the earth, which still acts upon the needle, and tends to draw it back to its natural magnetic meridian. Therefore it will come to rest in a place between the two forces, according to the predominance of the one or the other. The direction in which the deviation takes place depends upon two circum- stances : the first is, the position of the conducting wire of the battery in relation to the magnetic needle, as it may be above or under it, or it may pass it perpendicularly with an up-running or down-running current; the second condition is, the direction of the current. Ampere drew the attention of natural philosophers to the fact that the earth's magnetism prevents the magnetized needle from entirely obeying the influence of the electric current. To obvi- ate this he proposed the astatic needle. He first, or M. Nobili, constructed this by placing a second needle above, parallel with, and near to, say within a half inch of, the traversing magnetic needle. The upper needle, being made fast on the same centre shaft with the first, moves on the same pivot; but its polarity is reversed as to the other, so as to neutralize I _ its directive tendency in respect to the earth, ^^^^ and so that it shall remain at rest in any posi- Fig-15- Astatic Needle- tion. But the two needles cannot be perfectly alike in all re- spects, and possessed of absolutely the same quantity of magnetic 3p