126 PROCEEDINGS OF THE AMERICAN ACADEMY. 



free from this difficulty, and finally, that the white silk triple covering 

 of some of our wire is liygroscopic and that when a closely wound coil 

 of it is damp, there may be a very little leakage from turn to turn 

 through the insulation, under very strong excitation. In any event, 

 we have eliminated all error from these sources, and we believe that 

 the inductances of the standards we have finally used may be depended 

 upon to at least the twentieth of one per cent. 



The shapes of three of our standards are shown in Figure 2. In D, 

 the three larger plates (and the shaded cores) are of plate glass about 

 29 centimeters in diameter. The cores were mounted by Mr. Thomp- 

 son in an engine lathe, and were ground for about two days, under a 

 constant flow of soda water, b^*' a rapidly turning carl)orundum wheel 

 fastened to the tool post and driven by its own motor, while the lathe 

 moved slowly. In this way the plates were made very accurately 

 circular. A is also wound upon a plate glass spool, but the two coils 

 are wound together from two spools of wire triply covered with white 

 silk. P consists of two spools with plate glass ends, but the shaded 

 cores are ebonite rings. G shows a side ^•iew of P. 



The magnetizing solenoid consists of about 300 kilograms of triply 

 covered Number 10 copper wire wound uniformly with great care, by 

 Mr. George W. Thompson, upon a massive brass spool 186.2 centi- 

 meters long in inside measurement. The inner coil has 8117 turns 

 in 14 layers, and a resistance at room temperatures of about 7.7 ohms. 

 The outer coil, of slightly different wire, has 5872 turns in 10 layers, 

 and a resistance of about 9.8 ohms. The field intensity at different 

 points of the axis w^as found for a given current in each layer separately, 

 and it appeared from combining the results, that a current of one 

 ampere sent through the whole inner coil gives rise to a field of inten- 

 sity 54.71(3) gausses at the centre and 54.60(5) gausses at a point 

 50 centimeters from the centre, on the axis. A current of one ampere 

 sent through both coils in series creates an electromagnetic field of 

 intensity 94.19(5) gausses at the centre and 93.77(5) gausses at a 

 point 50 centimeters from the centre, a difference of nearly one half 

 percent. The outside diameter of the solenoid is a little less than 

 20 centimeters. 



For currents up to 31.5 amperes, corresponding to a field of about 

 2900 gausses, the coils were used in series attached to the 550 volt 

 circuit of the Harvard University plant. For stronger fields of 5000 

 gausses or more, the coils could be attached in parallel to this circuit 



