178 Transactions of tee American Institute. 



here see another vertical lantern, whose sides are made of thin 

 mahogany boards, so that the circumstances of this horizontal con- 

 densing lens, c, over which is placed the graduated circle, and the 

 magnetic needle may come as near as possible to the two vertical 

 spirals S and S, each formed of about fifty feet of -^ inch copper 

 wire. The two spirals, you observe, are so connected by a short cop- 

 per wire that a current of electricity revolves through both in the 

 same direction. Whenever this takes place, the needle which swings 

 between them instantly turns on its center, and from the direction and 

 amount of its rotation, we have the direction and amount of the 

 electric current. Under the needle is this inclined mirror, M, which 

 sends the rays fron^ this lime-light up through the graduated circle to 

 the lens, L, whence they issue to be reflected from the swing-mirror, 

 B,, on to the screen, where you now see the enlarged image of the 

 needle inclosed in the graduated circle. But a magnetic needle kept 

 in its meridian by the entire directive power of the earth would 

 require a strong amount of electricity to move it, and hence the use- 

 fulness of this lantern galvanometer would be very much restricted. 

 To render its needle more sensitive, we neutralize the earth's directive 

 action on it by means of these two large bar magnets, A and B, which 

 you observe on this board which forms the base of the lantern. The 

 poles of these magnets point in the same directions as the similar 

 poles of the needle, and thus they oppose the earth's directive action 

 on the needle. By approaching the magnets to the lantern the 

 needle is rendered more sensitive to the current, and by rotating one 

 or both of these magnets in a horizontal plane, we can immediately 

 balance the magnetic disturbances inevitably produced in the course 

 of these experiments ; such, for example, as happen whenever we 

 bring into action our great electro-magnet ; and thus, we have both 

 the sensitiveness and the direction of the needle entirely under our 

 control. "Without the aid of this instrument, the experiments which 

 I will shortly make could not have been exhibited to you, and I 

 therefore need hardly remark that this is the first time they have 

 been solved in a public lecture. It was bold to attempt experiments 

 of such finesse, but perseverance has crowned them with success, and 

 •it delights me to have the privilege of showing you their exquisite 

 beauty, which I will now proceed to do. 



You will notice that the two iron cores of our great magnet are 

 placed but to but, thus forming one bar of seven feet in length ; its 

 ends are terminated with those iron cores, which you observe pro- 

 jecting beyond the spools of wire which envelop all the rest of the 



