PEIRCE. — BALLISTIC GALVANOMETERS OF LONG PERIOD. 303 



PQ 

 < 



Table II gives some of the 

 results of several days' study 

 of the characteristics of the 

 galvanometer V. The periodic 

 time, which was determined 

 with the help of a chronograph, 

 is given in round numbers, 

 because slight differences of 

 dampness in the air or of 

 barometric pressure seemed to 

 affect the period somewhat. 

 With small values of R, the 

 ratio (r) of successive half 

 amplitudes was usually some- 

 what variable in the manner 

 described above, though the 

 values were persistent. Un- 

 der these circumstances the 

 average value is given. If 

 the instrument followed the 

 Gaussian law exactly, the 

 value of (3 should be the same 

 throughout. 



As this galvanometer was 

 to be used in an important 

 series of magnetic measure- 

 ments during which it was 

 necessary to determine with 

 accuracy the change of flux 

 in the solid core of a fairly 

 large electromagnet when the 

 exciting current should be 

 reversed in direction, it was 

 desirable to study with some 

 care the effect upon the throw 

 due to the duration of the 

 induced currents. If under 

 all ordinary cases the area 

 beneath the curve in the record of an oscillograph in series with the 

 galvanometer is proportional to the corresponding throw of the galvan- 

 ometer, one may assume that the performance of the galvanometer will 

 continue to be satisfactory ; but this test is not easy to make. It is 



