BABBITT. — THE VON WALTENHOFEN PHENOMENON. 235 



garded as sufficiently great to register within the accuracy of the in- 

 strument the flux change that took place. 



The throw was read by means of a telescope and scale ; the dis- 

 tance of the latter from the mirror was 86 cm. Since the total possi- 

 ble deflection on either side of the zero amounted to 23 or 24 cm., it 

 was necessary either to compare angles calculated from the deflec- 

 tion or to compute the angles for one set of deflections, add them, and 

 determine the single deflection corresponding thereto. In other words, 

 for throws of more than ten centimeters it was not admissible to regard 

 the deflection as proportional to the flux change in the toroid. 



The calibration by means of the standardizing solenoids showed that 

 the throw could be obtained consistently, for any given quantity of 

 electricity sent through the instrument, to within 0.1 mm. Thus for 

 throws greater than 10 cm. 0.1 per cent and often 0.05 per cent was 

 the accuracy attained. For such agreement three things were abso- 

 lutely essential : the suspended system must be at rest before the 

 throw commenced ; the throw must be taken always in the same direc- 

 tion ; the coil, in coming back, must never swing more than a few 

 millimeters beyond its zero position. If it did go far beyond, the zero 

 point was changed and the next deflection or two were not quite what 

 they should be ; usually too large. In this connection, too, it was ob- 

 served that the first two and sometimes three throws taken in the day's 

 work were in general larger than those that followed by a fraction of a 

 millimeter ; they were always thrown out of each set of results as 

 doubtful. Although the care of complying with all the conditions 

 made it necessary to spend from three to six hours in taking the data 

 for any one of the curves presented later the accuracy of the results 

 seemed to make it worth while. 



One more peculiarity came out during the investigation. When the 

 current in the primary of the calibrating solenoid was simply broken, 

 instead of reversed as usual, and the deflection for the reversal calcu- 

 lated from the resulting throw, this was in general a few tenths of a 

 millimeter greater than an observed deflection for a reversal. The 

 diff'erence appeared only when the total reversal gave a throw of more 

 than 12 or 14 cm. It seemed as if somewhere in the secondary circuit 

 there must be a slight leakage produced by the higher voltage of the 

 complete reversal. In order to be sure of the amount of this differ- 

 ence a full reversal was taken in the''calibrating test before each day's 

 work as nearly as possible equal in throw to that produced by the re- 

 versal of the field to be used on the toroid during the day ; then the 

 throw was observed when the same current was broken, and the error 

 calculated. It was hoped that the conditions in the secondary circuit 



