232 PROCEEDINGS OF Till-: AMERICAN ACADEMY. 



S= 1.489 



D = L.905 cms. 



n = ,',<> turns 



B_ loo It 



We get T " Z 2(1.489) 77 (0.9525) 8 • 51 1 * 



The right-hand member is a constant for any given /.'. In the work on 

 the series of curves the I! had values ranging from 1 17 to 7117 ohm- : 

 the galvanometer and secondary coil circuit having itself 1 17 ohms, of 

 which the galvanometer had about 99 ohms, and the coil 18 ohm-, 

 the other resistance being added, when convenient, from the resistance 

 box It'. The constants for these various R'a were found and written 

 down. Then all we have to do to find the B for any observation is t«. 

 multiply the observed throw in centimeters by the proper constant. 

 This was done either by means of logarithms or a very good slide rule. 

 If we use the step-by-step method, the formula simply drops the 

 factor 2 and becomes, 



Ml lnii// 



/• &r(Z> 2) a n 



For the long solenoid we have simply 



i — v 

 //' = T (No. of amperes used) 

 \.{jJj 



= --^.tiG-i (No. of amperes). 



Having found the values of B and //', they were multiplied by I 

 ui'l 2 respectively, in order to facilitate the plotting of the points of 

 observation. Then the magnetization curves were drawn by free-hand 

 so as to fit the points as closely as possible. 



This gives us the curves from m= r> to 200 in Figure 1">. To find 

 the corresponding normal curve (in = *-) a graphical device was found 

 to he of the very greatest utility. Nol only was an enormous amount 

 if time saved, which otherwise it would have been necessary to >pend 

 in almost endless computations, but the device was a positive aid in 

 determining the position of the normal curve. On a large sheet of 

 tracing cloth were drawn about seventeen horizontal lines, so that when 

 properly placed over the sheet of millimeter paper on which the mag- 

 netization carves had been drawn, they coincided with the lines Z? = 0, 

 1000, 2000, etc, up to 16,000. By means of lines radiating out from 

 a point on the lowest of these horizontal lines, each one of the Hi 



