236 PROCEEDINGS OF THE AMERICAN ACADEMY. 



■when the current in the solenoid was reversed would be sufficiently 

 like those existing when the field of the toroid was reversed to give 

 the same leakage. As a matter of fact, the difference between the cal- 

 culated and observed reversals for the solenoid were throughout so 

 nearly equal to the corresponding difference when a reversal consisting 

 of a number of steps was compared with a complete reversal of the field 

 in the iron, that in all probability the conditions were approximately 

 the same and the leakage likewise. Unfortunately this leakage was not 

 taken into account at first because of the other greater inaccuracies 

 then occurring in the work ; it is applied, therefore, only to the work 

 done on coils I and III, not to 11. 



The amperemeter was supplied with a number of shunts so that the 

 constancy of the current could always be determined to within 0. 1 per 

 cent, and as a general rule more closely than that. It did not appear 

 to be necessary to measure so exactly the true value of the current 

 used ; and no attempt was made to do so. However, it was given by 

 the instrument within 0.5 per cent. 



The resistances in the primary circuit were made of constantan wire 

 set in enamel. For the moderate currents used, seldom over two am- 

 peres, their rise in temperature did not have any appreciable effect. 



The primary and secondary solenoids used in calibrating the ballis- 

 tic galvanometer were both carefully made. The primary consisted of 

 5526 turns of No. 18 B. & S. annunciator wire wound on a brass tube 

 split longitudinally. The total length of the coil was 176.2 cm. The 

 secondary consisted of 538 turns of well insulated wire wound on a 

 wooden core. The primary was attached to the wall vertically at a 

 distance of 15 feet from the galvanometer ; inside it and equidistant 

 from either end was hung the secondary. This arrangement being 

 always constant allowed the testing of the galvanometer at the begin- 

 ning and end of the day's work, and showed immediately whether any 

 change had occurred. 



Except for soldered joints the secondary was throughout a copper 

 circuit. Thus all trouble Irom thermoelectric currents was eliminated. 



Three toroids were used which for convenience we may designate as 

 I, II and III. The second was the first to be experimented on, but the 

 meaning of its data will be clearer if discussed after the more extensive 

 investigation with toroid I. 



The core of I was made out of carefully annealed soft wire. After 

 the winding, the wires were insulated from each other by soaking the 

 whole in shellac and drying it. A layer of paper covered the core ; 

 on it was wound a single layer of secondary divided into four parts. 

 A second layer of paper, the first layer of the primary, a third layer of 



