E ledro- Magnetic Apparatus and Experiments. 261 



that primitive current. By derivation is meant, the addition of 

 another wire to any portion of the primitive circuit. The simple 

 sohitian of the fact is, that derivation, or the addition of another 

 wire, increases the conducting power of the circuit. Professor J. 

 Henry's discovery of the method of increasing the power of the 

 electro-magnet by winding upon it several short coils of wire, is 

 a most striking practical illustration of this law. M. Pouillet has 

 also arrived at the conclusion, that the intensity of the current 

 produced by a single element, is in an inverse proportion to the 

 real length of the circuit. The adoption of the several strands 

 in the galvanometer seems therefore to be plainly indicated, and 

 experiment fully warrants it. b, fig. 2, is a graduated circle of 

 ivory for marking the deviations of the needle. Since the con- 

 struction of the instrument, fig. 2, I have adopted the plan rep- 

 resented in fig. 3, which is much to be preferred on account of 

 its simplicity of construction, and the perfect steadiness of the 

 needle, c, is the coil cemented upon the stand d ; b,si graduated 

 zone surrounding the coil, p and w, the wire terminations, a, 

 the circular needle of watch spring, with a very delicate upper 

 bearing at c, and a slender pivot at a, resting upon an agate centre 

 cemented to the coil. As this needle is not liable to any mechan- 

 ical displacement, it may come very near the coil c. The por- 

 tion of the circle between the two lines at a, which bears the 

 pivot, is of brass. 



Double Helix for Inducing Magnetism. 



Figure 4, represents an apparatus contrived January 11th, 1838, 

 for exhibiting the magnetic forces of the centre of the helix, a, a, 

 are the two helices of five layers of wire, protected by brass ca- 

 sings, (split on the under side,) and by ivory heads, c,c.c,c. b, b, 

 are two curved bars of soft iron which slide readily into the he- 

 lices. 0, 0, the handles for pulling, furnished with ball and socket 

 joints at o, o, to prevent the magnets being twisted or wrenched. 

 The wire terminations of the helices pass through the openings 

 in the brass casings, underneath the base board, and are soldered 

 to the screw cups p, n, for battery connexions. The attractive 

 force manifested by this arrangement in the centre of the helices, 

 is much greater than when an armature is applied at the extrem- 

 ities. A small apparatus of this kind will resist the strength of 

 two stout men pulling by the handles. This makes a very pretty 



