160 



Two primary objects were sought in modifying the Thompson Permeameter 

 as shown in Ewing's work, Fig. 130, page 249: First, to render the instrument 

 more accurate, and Second, to avoid complexity such as is necessary in the 

 ballistic method so that the magnetic properties of iron may be introduced 

 earlier in a students' course. 



The modified apparatus is shown in Fig. 1. The solenoid (A) is sur- 

 rounded by a cast iron field (B) which furnishes a metal path for the return 

 lines of force. The lug (L) is separated from the core of the solenoid by a 

 very thin piece of paper. The core of the coil can be easily replaced by a 

 core of a different metal, thus giving a difTerent test. The force required to 

 separate the lug from the core is measured by means of the spring (S). Since 

 the pull exerted by a spring is directly proportional to the distance it is 

 stretched it becomes necessary simply to measure accurately the distance 

 the spring is stretched in separating the lug (L) from the core. The upper 

 end of the spring is attached to a sliding guide bar (G), to which is fastened 

 a micrometer screw (T). By turning this micrometer screw the spring may 

 be stretched sufiiciontly to pull the lug away from the core which is being 

 tested. The number of whole turns of the screw may be read from the index 

 bar (D), and the fractional part of a turn to one-tenth of a turn may be read 

 from the disk (E.) By standardizing (lie spring and reading micrometer by 

 means of known weights the force in screw turns may ])e reduced to grams 

 or pounds. 



The permeameter with the auxiliary apparatus is set up as shown in 

 Fig. 2. 



-VVWWV\A- 



t 



:£ 



M 



J^tf.2.. 



J', is the permcaineter. 



C, i.s rotary commutator by mean.s of which an alternatinK currvnt may l)e tlirown tlirough 



the solenoid, thus demagnetizing tho core. 

 R, is a variable resistance by means of which the current may I)i' varied from zero to twenty- 

 five amperes. 

 A, is an ammeter. 

 M, repre.sents the .source of current, which is the ordinary 110 volt direct lightinp; current. 



