Galvanic Coils. A cheap and effective galvanic coil for invalids may be con-structed as follows. - AB is a rectangular pi ece of board, upon which is fixed vertically a hollow cylinder of wood, CD, wider at the bottom, like a reel, but not quite so wide at the top. Inside this cylinder is dropped a piece of bar iron, E, which rises slightly above its surface. On one side is a brass upright, F, from which proceeds a piece of watch-spring, II, having at its end a small cone of iron which nearly touches the bar, E. On the opposite side is an upright of brass, G, from which proceeds a flat piece of brass, I, through which passes a brass screw, J, having at its extremity a small piece of platinum, which comes in contact with another small piece of platinum fixed on the watch - spring, H. Round the wooden cylinder, CD, is wound some covered copper w:ire (No. 12 or 14) nine times (more or less as the number of powers required) at right angles to its axis, so as to constitute nine helices, which are connected, and together form one helix continued within itself. The interior is the first, the exterior the last helix. The winding of the wire should proceed from the top of the cylinder, leaving a piece about one foot in length, which, pass ing down laterally between the i the first helix and the wooden cylinder, and emerging at bottom, is pushed through a hole at K, passed under the board, and connected with the bottom of a binding screw, L. The last coil of the first helix is then laid bare, by removing the covering away a little at each side; and to this is fixed a piece of uncovered copper wire, which is pushed through a hole at a and connected under the board with an upright piece of brass wire, 1. The top of the second helix is next laid bare, and connected in a similar manner with the upright piece of brass, or power, 2, by a piece of copper wire passing downwards between its interior and the exterior of the first helix, and through a hole at b. And so from top and bottom alternately, are the remaining helices connected with the other powers; thus, the bottom of the third helix through the hole at c, with the 3rd power; the top of the fourth, etc.; until the top of the eighth is connected with the 8th power. Then the exterior end of the continued helix, which is the bottom coil of the ninth helix, is passed through a hole at i, and connected with the bottom of power 9 ; which power is also connected, by a wire passed upwards through a hole at j, then along the outside of the helix and through the top of the cylinder, with the bottom of the upright G. The binding screw N is similarly connected, through a hole at M, with the upright F. The wire which passes under the board from N to M. is joined in the middle to another piece of wire connected with the bottom of the binding screw S. Between the cylinder and the powers is fixed a piece of brass wire, OP, bent twice at right angles, the bottom of which is connected with the binding screw Q. This brass wire has a moveable piece of brass, It, which may glide along it, and rest on either of the powers required. The coil should be covered with leather, the bottom of the board with baize. Care should be taken that there be no metallic contact any where, except in the parts mentioned. The coil is then complete; its expense is 16s. L and N are screws for battery, S and Q for patients. In galvanizing invalids it is a matter of primary importance that the cross current should have the same direction as the nervous fluid. It is a forgetfullness of this which renders futile the attempts of many galvanists. Again, the instruments sold by philosophical instrument makers have generally two continued helices, in one of which the voltaic circuit is completed, in the other a current is induced, which is atterlv useless as a remedial agent.