114 PROCEEDINGS OF THE AMERICAN ACADEMY. 



sals is usually unsatisfactory with large cores. A set of adjustable 

 electrolytic resistances fitted for carrying heavy currents is often 

 useful. 



In the case of a very large closed electromagnet, even if the core be 

 laminated, it is extremely difficult to get very useful results by aid of 

 a ballistic galvanometer of short period, but if one has a suitable oscil- 

 lograph or other recording instrument at hand, it is easy to obtain a 

 diagram something like that shown in part in Figure 9, though it is 

 necessary to make sure that the intervals between the steps, unlike 

 those in this figure, are long enough to record the whole of each in- 

 duced current. 



If the primary current (/, t) curves are to be used in studying the • 

 magnetic changes in the core of an electromagnet, the sensitiveness of 

 the oscillograph must be so adjusted that the deflection due to the 

 largest value of the current {U, Figure 4) will make a record on the 

 paper ; if the (/', t) curves are to be used, the steps may be as numer- 

 ous as one likes, and the sensitiveness of the recording instrument may 

 be so great that, starting from the base line, the record of the highest 

 induced current shall just fall on the drum. In this latter case the 

 areas to be measured may be made so large that any uncertainty as to 

 the exact time when any induced current may be considered to end is 

 unimportant. When many records are taken on the same paper, the 

 drum has an opportunity to revolve a good many times during the 

 operation, and it is not always easy to decipher the complicated maze 

 of curves. Of course the fact that an electromagnet has a closed secon- 

 dary circuit modifies somewhat the form of the building-up curve in the 

 primary, but, theoretically at least, this should not affect the value of 

 the magnetic flux due to the primary current if its final intensity is 

 given, and the difference is inappreciable if there are only a few turns 

 in the secondary coil. 



Instead of changing the resistance in the primary circuit suddenly, 

 at each step. Dr. Thornton, in dealing with the frames of some very 

 large dynamos, made each step gradually, by moving an electrode 

 slowly in a trough of acidulated water from one stopping place to 

 another. Figure 10 is a close copy of one of his records published in 

 the " Philosophical Magazine " for 1904. 



FlUXMETERS AND QUANTOMETERS. 



Given an amperemeter of the ordinary d'Arsonval type, in which an 

 open-frame, low resistance, unshunted coil swings in the strong mag- 

 netic field between an interior soft iron core and the hollowed-out jaws 

 of a powerful magnet, it is often possible to make the controlling 



