Maech 16, 1917] 



SCIENCE 



269 



Table 2 and Fig. 7 is an exhibit of tbe data 

 obtained when the plane of the pendulum vi- 

 bration passed through the plane of the coils, 

 X changing from negative to positive values. 

 Unfortunately the undamped needle does not 

 stop vibrating when the intensity of the induc- 

 tive impulse is reduced to zero; otherwise the 

 rotation of the earth might be directly read 

 off at p, Fig. 6, by rotating the coil on a tan- 

 gent screw. The reduction factor i'' in ^ = Fx 

 was measured for 3 arcs : At D ^ 24 cm., 

 ^=.054°, at D = 14 cm., ^=..087° and at 

 D = 8.7 cm., (9^.111°, corresponded respec- 

 tively to a; = 1 cm. 



Electrical Pendulum Registry 

 6 = Fx — .5° cylindrical magnetic bob, length 

 20 cm., diam. 2.5 cm., mass 800 grams. Long 

 square coil, 6 layers, 34 turns each, 30 cm. long, 5 

 cm. broad, 5 cm. high within. Bob and astatic 

 needle of galvanometer with synchronized period 

 of 4 sec. 



For other arcs D the reduction factor F was 

 interpolated. When x is negative, the arcs cc 

 are in excess of the electromotive impulses 

 which are decreasing toward zero. When x is 

 positive the arcs are in deficiency of the in- 

 creasing impulses due to the rotation of the 

 earth. Hence an undamped needle does not 

 come to rest and in Table II. and Fig. 7, 

 ^^0° at t = 2 min. was interpolated (paren- 

 thesis) from the subsequent 8 data. This 

 makes 6 = Fx — .5°, beginning with i^5 

 min. The fluctuations of $ are due to the 

 rough measurement of D and the correspond- 

 ingly rough value of the reduction factor F 

 and are quite as good as anticipated. Even- 

 tually the decrement of x due to decreasing 



arc D must begin to approach the increments 

 due to the earth's rotation, whereupon x will 

 be stationary. This seems to happen after 

 45 m. in Table II. 



Again if the reduction factor i^ of a; is taken 

 constant throughout, the results show the 

 rapidity with which the values fall off even 

 after 10 minutes. Thus it seems that a com- 

 pound penduliun on knife edges, Fig. 4, with 

 the magnetic bob similarly placed to the coil 

 must be used for standardization. 



In other series experiments the reduction 

 from X to was made linearly, the constants 

 being a mean approximation from a direct 

 measurement of x and 6- This however is the 

 real difficulty of the method and is far from 

 satisfactory owing to the development of cross 

 vibrations. 



In the final results the case of a core of 4 

 iron plates (each 18 cm. X 25 em. X -044 cm.) 

 placed symmetrically within the coil was 

 tested. In view of the breadth of these plates 

 and the weight of the pendulum there was 

 supposed to be no danger from induction. 

 The sensitiveness (scale at 4 meters) was thus 

 increased to an initial growth of a; = 5 cm. 

 per minute of earth rotation. It would have 

 been larger if the periods of pendulum and 

 needle had been as nearly the same as before. 

 Here I found roughly e = Fx={.\lQ — 

 .0035Z>)a; and it was interesting to note that 

 for the last data the term in Dx had passed 

 through a maximum. Hence the increments 

 of x are much reduced. If the logarithmic 

 decrement is used, ^ = 60(a — hD„c*/^)x de- 

 grees per hour, follows, where a and h are 

 the constants given, D = 27, c = .896. Greater 

 smoothness is thus obtained, but the real diffi- 

 culty which resides in the constants a and i 

 is left untouched. Finally one may note that 

 the data with a plate iron core in the coil were 

 apparently as good as those obtained without; 

 for the correction coefficients which indicate 

 the growth of cross vibrations were actually 

 larger (accidentally) in the absence of iron. 



8. Short Pendulum. — The endeavor was now 

 made to use the same method for a short 

 pendulmn. For this purpose the magnetic 

 cylinder was swung on a round glazed fish line. 



