Scientific Lectures. 177' 



of Copenhagen, in 1819, and since then many instruments called 

 galvanometers, that is, measures of galvanic currents, have been 

 devised by applying the following facts. 



You observe on the screen the magnetic needle, supported on its 

 steel point, around which it turns with the greatest ease. I will now 

 bring quite close over it this copper wire, and stretch it in the direc- 

 tion of the needle, north and south. You now see on the screen the 

 wire, one end only of which is at present connected with this small 

 galvanic battery ; when the other end is likewise connected, an elec- 

 tric current will pass, as we say, through the wire, and the needle 

 will move. I now join the other end of the ware to the battery ; 

 look how the needle swings around, tending to plrce itself athwart the 

 wire. If I place the wire below the needle, observe it swings in the 

 opposite direction. Also, if the wire remain either above or below 

 the needle, and I reverse the current, then the direction of rotation 

 of the needle is reversed also. Therefore, if we pass a wire around a 

 magnetic needle — enclosing it many coils — we have a most precious 

 instrument, in which the direction of the needle's deflection shows the 

 direction of the current, and the amount of the deflection shows the 

 amount of the current. 



But, as you have observed in these experiments, the wire is either 

 above or below the needle, and to render the latter sufficiently respon- 

 sive to an electric current, the wire has to envelop it with many coils ; 

 but such an arrangement will completely hide from sight the motions 

 of our lantern needle. How, then, can we show its motions to so 

 large an audience ? The idea of using a galvanometer in this lecture 

 weighed heavily upon me after I had decided to work before you, for 

 heretofore the usual method of exhibiting the motions of a galvan- 

 ometer needle consisted in attaching to it a light mirror, and observ- 

 ing the motions of a beam of light reflected from it on to the screen. 

 A faint patch, or brighter band of light, on a screen, and the inadvert- 

 ent approach of somebody's jack-knife, or the handling of some dis- 

 tant magnet, in an experiment, and this patch of light is ofF the 

 screen and probably brought again to subject itself to our service, 

 after your patience and my lecture's continuity are lost. It w T as evi- 

 dently imperative that a new method of exhibition should be devised, 

 and the result of the investigation is this galvanometer, which keeps 

 the image of the needle itself always in full view, and exhibits and 

 measures its smallest swings as its ends course round the divided 

 circle you now see on the screen. 



The plan of this instrument can be explained in a few words. You 



[Inst.] 12 



