494 



PROCEEDINGS OF THE AMERICAN ACADEMY. 



the motion of which is controlled by a small magneto-motor driven by 

 three Leclanche cells. The motor was governed by a reversing key 

 in the testing room, about one hundred feet away. The alternator 

 used had ten poles, thus making the length of a wave 72°. A piece 

 of brass, 36° or one half a wave long, was set into the edge of the disk, 

 and the whole carefully trued up. When the dynamo was in action 

 the B current was closed once during a revolution, and for a time cor- 

 responding to one half a wave length. The galvanometer then per- 

 forms the operation ^ ^ 



fifteen hundred times a minute. The value of this integral will be a 

 maximum when it extends over the time between -{-B and — B. By 



the use of the key K^ the brushes may be shifted, and the reading of 

 the galvanometer brought to its greatest value. With the dynamo 

 running at its normal speed a Leclanche cell, E.M.F. 1.4 volts was in- 

 serted in the circuit ; Rh was 1,374 ohms, and the deflection 21.2 cm. ; 

 with brushes short circuited and Rh equal to 14,874 ohms the deflec- 

 tion was 21.8 em., the ratio of deflection being .972. If the brushes 

 made perfect coutact, and there was no self-induction effect, the ratio 

 would be unity, the entire cycle of operation being performed in a time 

 short compared with the period of the galvanometer. During the tests 

 the reading with R h = 1,374 was frequently repeated in order that 

 the brushes might be kept in as uniform a condition as possible. 



Owinfi; to the manner in which the specimen is constructed, the area 

 included in the B winding is but partially occupied by the iron. In cal- 

 culating the approximate values of B no allowance has been made for 

 this, as the permeability is high in the cases where B has been deter- 

 mined. If observations were taken at temperatures where the permea- 



