496 PROCEEDINGS OF THE AMERICAN ACADEMY. 



the magnetizing coil was the B coil of 24 turns. These two windings 

 formed on the outside of the specimen a single layer. The tempera- 

 tures were determined by a Le Chatelier pyrometer, calibrated by 

 known melting points. The hot junction was placed inside the asbestos 

 wrapping resting against the iron, the leads were brought out through 

 the usual double-cored fire clay tube ; this was surrounded by a piece 

 of gas pipe thoroughly wrapped in asbestos cloth, because in using this 

 pyrometer in a case like the present, where the leads pass from the 

 hot junction through a space at a higher temperature at a short dis- 

 tance from the junction, it is necessary to guard against conduction 

 along the leads, and consequent errors in the temperature. 



The manner of making the tests and the subsequent calculation was 

 as follows. Owing to the construction of the furnace, the specimen 

 being well shielded from the heat of the fiames by the internal chamber, 

 the air insulation, and its own asbestos covering, the temperature of the 

 iron rises gradually ; consequently the specimen will be at very nearly 

 a uniform temi^erature. When a reading is desired, the alternating 

 current is turned on and the S deflection adjusted to< its maximum 

 value by J^^, and to its correct numerical value by varying the mag- 

 netizing current. Watt-balance resistance r^ is now adjusted, and the 

 pyrometer reading corrected for zero, and dynamometer reading and 

 time observed ; after a few minutes the observations are repeated, the 

 temperature having risen in the mean time. This process is kept up 

 until the iron loses its magnetism. Occasionally, a direct current was 

 sent through the apparatus and the Watt-balance, dynamometer, and 

 pyrometer readings taken ; these allow the value of r-^^ in the hysteresis 

 formula to be determined by making a plot, the co-ordinates being 

 Watt-balance and pyrometer readings, from the plot the value of 

 ri\ corresponding to any pyrometer reading may be read off. The 

 electrodynamoraeters were calibrated by sending a direct current 

 through them, and measuring the P.D. at the terminals of a known 

 resistance by projection against the E.M.F. of a Clark cell ; the values 

 of deflections and currents squared were plotted, giving straight lines. 

 j?^ was determined by reference to these plots. The zei'o errors of 

 the dynamometers employed were very variable, and were determined 

 after each reading. 



The results of the measurements will now be given. They are 

 plotted as referred to at the beginnings of the several tables. 



