206 PROCEEDINGS OF THE AMERICAN ACADEMY. 



Length = 207.7 cms. 

 Outside diameter = 5.97 cms. 

 Inside diameter = 3.63 cms. 



This solenoid was wound on a tube of pasteboard with two wire coils 

 of 3386 turns each, — of No. 18 wire, in six layers, — which were used 

 in parallel, so that 



H' = A-)iC/10 = 20.5 • (No. of amperes used). 



Later on in the work a still longer solenoid was built, in order to ex- 

 periment on very thick iron rods. A is a " P-3 " amperemeter, that 

 is, one of the type so successfully used in the laboratory of the course 

 Physics 3 in Harvard University ; it reads with great accuracy up to 

 1.5 amperes. K is a double reversing knife switch, connected to the 

 solenoid S, and also tO a demagnetizing solenoid D, with an iron core 

 in the small coil, which could be connected to the light circuit L. R 

 is a rheostat in series with a system of variable resistance coils, to 

 regulate the current. P is a reversing key to change direction of 

 ballistic throw in the galvanometer, 7^ is a tapping key arrangement 

 with small ^battery, for bringing the galvanometer magnet needle to 

 rest. Its circuit contains a very high resistance W. Z is the galva- 

 nometer scale with telescope, at 116 cms. distance from magnet 

 system. II' is a resistance box in the secondary circuit ; by varying 

 this resistance the throws were kept under control, so as to give good 

 accuracy in the readings. 



The " P-3 " galvanometer was frequently compared with a Weston 

 milliamperemeter with shunt, and the sensitiveness of the galvano- 

 meter was often determined during the course of the work by charging 

 a condenser of one microfarad capacity from a battery of four Samson 

 (wet) cells whose voltage was read off on a voltmeter. The sensitive- 

 ness, given in centimeter divisions of throw per coulomb, ranged from 

 1.24 to 1.60. In the latter part of the work the condenser was 

 charged by connecting across a standard resistance of 10 ohms, say, 

 through which about 1 ampere was flowing, thus getting about 10 

 volts. 



In the earlier half of the experiments the "reversal" method was 

 used with great convenience and accuracy in the readings. The 

 magnet suspension does not hold. its zero very closely, but is slowly 

 tossed about by magnetic disturbances over a range of 1 mm. scale 

 reading, and sometimes more. ]\Ioreover, the zero position, which is 

 quite definite at any one time, often changes slowly during the course 



