446 TRANSACTIONS OF THE AMERICAN INSTITUTE. 



three, four, five, or the number of pah's used, the resulting deflec- 

 tion was marked on the scale, and this established the relation 

 for all eases. To carry the graduation through the whole quad- 

 rMit required several series of plates. This method was only- 

 applicable to slight currents, such as were required in making 

 very delicate experiments with bodies but slightly heated. The 

 {^sumption that the current excited b}^ heat would be uniform, 

 and that the increase would be exactly in proportion to the num- 

 ber of heated joints, was not precisely true. 



Nobili's method was to fix his graduation of the scale by differ- 

 ences. He assumed that the deflection of his galvanometer was 

 proportional to the force of the current for the first twenty 

 degrees, and he determined the graduation beyond this point by 

 attaching to the galvenometer a thermo-pile and warming one end 

 of the pile by bringing a spirit lamp near enough to it to cause a 

 deflection of twenty degrees in the needle of the galvanometer. 

 The pile was then screened until the needle returned to the zero 

 point. 



The next allowed the heat of a second lamp to fall on the other 

 side of the pile, which produced a deflection of the needle in the 

 opposite direction ; the heat was regulated so that a deflection of 

 twenty-four degrees was made. Both lamps were then allowed 

 to act simultaneously on the pile, and he obtained by difference 

 of the currents, a deflection not of 4° but of 5°.l. He therefore 

 concluded that the current which produced a deflection of 24°, was 

 the result of 20-f-5-l-» or 25.1. units. In this way, by increasing 

 the activity of the pile, he determined 28°-24, 32°-28, and so on, 

 and afterwards filled in the intermediate degrees by the eye. 



Nobili had tried this principle with a galvanometer having two 

 wires connected with two batteries. He passed currents from 

 them in opposite directions, first separately, and then together. 



PoggeudorfF proposed to use one current, of uniform strength. 

 The principle applied by him, is thus expressed in his own words, 

 as translated by Dr. John D. Easter : '• The deflections produced 

 by currents of different strength, passing through the coils of a 

 multiplier lying in a magnetic meridian, can be deduced from those 

 produced by one and the same current, passing through the same 

 coils, at various inclinations, to the magnetic meridian." The 

 possibility of this is shown by geometrical considerations. Pog- 

 gendorff proceeds to illustrate, by a figure, the force of the mag- 

 netism of the earth, tending to draw back a needle after having 



