58 PROCEEDINGS OF THE AMERICAN ACADEMY. 



employed in our work. Some of our observations are recorded 

 briefly here in the hope that they may prove useful to other experi- 

 menters. 



Figure 1 shows the general arrangement of the apparatus used in 

 making the measurements described in this paper. Figure 2 indicates 

 the forms of three of the several standards of mutual inductances 

 which we used for calibrating the galvanometers. For a complete 

 discussion of the apparatus used throughout this work reference is 

 made to a preceding paper, 3 since the apparatus used for both investi- 

 gations was essentially the same. 



Our method of work was not new in any particular, but the necessity 

 of using heavy currents and, therefore, of taking care of the heat 

 equivalent of many kilowatts in our circuit, and the use of ballistic 

 galvanometers with periods so long that a reversal of current in the 

 highly inductive circuit could be accomplished before the galvano- 

 meter coil had moved appreciably from its position of rest, introduced 

 many difficulties which could be overcome only after much anxious 

 experimentation. 



The large solenoid with which most of our work at high excitations 

 was done, was made of about 300 kilograms of triply covered No. 10 

 copper wire wound uniformly, with great care, by Mr. George W. 

 Thompson, the mechanician of the Jefferson Laboratory, upon a 

 massive brass spool, 186.2 centimeters long, in inside measurements. 

 There are two coils, one of 8117 turns in 14 layers, and the other, of 

 slightly different wire, of 5872 turns in 10 layers. The field intensity 

 in the centre of the solenoid when a current of one ampere passes 

 through the first coil is 54.71 gausses, while at a point fifty centi- 

 meters from the centre on the axis, the intensity is 54.60 gausses. A 

 current of one ampere sent through both coils in series creates a field 

 of 94.19 gausses at the centre of the coil. This solenoid was repeatedly 

 tested for leakage between the turns by means of a very carefully made 

 test coil without iron, but we were never able to detect any evidence 

 of fault. 



For low excitations, we sometimes used a somewhat longer solenoid 

 wound with No. 14 wire. 



The test pieces were first packed in fine iron filings in a pipe closed 

 at the ends by caps. This was placed horizontal and perpendicular 

 to the meridian upon supports in a furnace where it was exposed to 

 several hundred gas jets driven by a power compressor. The dimen- 



3 Peirce, These Proceedings, 49, 1913. 



