130 
the current from a galvanic trough is capable of producing greater 
magnetic effect on soft iron after traversing more than one-fifth of a 
mile of intervening wire than when it passes only through the wire 
surrounding the magnet.’’ In speculating on a result apparently so 
paradoxical, Henry suggests that the ‘‘ current from a trough pos- 
sesses more ‘projectile’ force (to use Prof. Hare’s expression) and 
approximates somewhat in ‘intensity’ to the electricity from the 
common machine.’’ ‘* But be this as it may,’’ he concludes, ‘‘the 
fact that the magnetic action of acurrent from a Zrvovgh is at least not 
sensibly diminished by passing through a long wire is directly appli- 
cable to Mr. Barlow’s project of forming an electromagnetic tele- 
graph ; and it is also of material consequence in the construction 
of the galvanic coil. From these experiments it is evident that in 
forming the coil we may either use one very long wire or several 
shorter ones, as the circumstances may require; in the first case, our 
galvanic combination must consist of anumber of platesso as to give 
‘projectile’ force; in the second, it must be formed of a single 
pair.” * 
In 1832, Henry described the production of electrical effects 
from the action of magnets.t Having wound upon the middle of 
the soft iron armature of his large electromagnet an insulated 
copper wire about thirty feet long, he observed that whenever the 
magnet was charged by the battery current a deflection of about 
30° to the west took place on a galvanometer connected with the 
ends of this wire. This deflection was but momentary, however, 
the needle returning to zero, although the magnet remained excited. 
On opening the circuit, a momentary deflection to the east took 
place. ‘‘ From the foregoing facts,’’ he says, ‘‘it appears that a 
current of electricity is produced for an instant in a helix of copper 
wire surrounding a piece of soft iron whenever magnetism is in- 
duced in the iron, and a current in an opposite direction when the 
magnetic action ceases; also that an instantaneous current in one 
or the other direction accompanies every change in the magnetic 
intensity of the iron,’’ 
It was while engaged in these experiments that Henry observed 
the phenomena due to the induction of one portion of the wire 
upon another, now called ‘‘self-induction.’’ He says: ‘‘ When a 
small battery is moderately excited by diluted acid, and its poles 
* Amer. Jour. Science and Arts, xix, 403, 404, Jan., 1831. 
+ Amer. Jour. Science and Arts, xxii, 403, July, 1832. 
