238 PROCEEDINGS OF THE AMERICAN ACADEMY. 



The demagnetization of toroids I and II was carried out in one of 

 two ways. In one process the toroid was left with its primary in the 

 primary circuit, Figure 3. By means of the reversing switch the field 

 was reversed and after each reversal decreased by the addition of resis- 

 tance from the rheostat and the gaps provided for that purpose. When 

 the current had been reduced to about 0.1 of an ampere the toroid was 

 disconnected and attached to the secondary of an alternating current 

 transformer. This was movable and could be pulled along a track 

 away from the primary which was connected to the 110-volt, 60-cycle 

 lighting circuit. The further demagnetization was carried out in this 

 manner. 



The second method consisted of the introduction into the primary 

 circuit, Figure 3, of a piece of apparatus designed to reverse the cur- 

 rent automatically. The main features were a motor and four rods 

 joined to the motor through gears and cranks. The cranks lowered 

 the rods by pairs into mercury cups beneath. With one pair of rods 

 in contact with the mercury, and with the proper electrical connections 

 from the toroid to the rods, and from the mercury cups to the battery, 

 the current flowed in one direction through the toroid ; when the 

 motor was turned until these two rods were raised and the other 

 pair were in contact, the current flowed in the opposite direction. The 

 running of the motor together with the introduction of resistance into 

 the main circuit resulted in the demagnetization of the toroids. By 

 keeping the amperemeter in the circuit it was possible to regulate the 

 demagnetizing current so that it should never be above that already 

 applied to the toroid. This is a distinct advantage over the first 

 method, in that the history of the iron is better known and the effect 

 of its previous magnetic history more easily determined. With the 

 motor running so as to give three or four reversals of the field in the 

 toroid per second, the resistance was introduced gradually from the 

 rheostat and gaps until the current was not above 0.0003 ampere. 

 The time given to one demagnetization by this device was between 

 two and three hours. 



The Investigation of Toroid I. 



The primary and secondary of the toroid were connected with their 

 respective circuits as shown in Figure 3. If M and the other switches 

 be set so that the current flows through K, and if, after the core has 

 been demagnetized, M be thrown over, the throw of the calibrated gal- 

 vanometer will give means of calculating the B induced in the iron. 

 Let the current be reversed by while the secondary circuit is broken 



