682 



TEXT-BOOK OF PHYSIOLOGY. 



tact with a muscle, the induced current will give rise to contractions which 

 will succeed each other so rapidly that they fuse together, producing a 

 spasm or tetanus of the muscle. For this reason these currents are fre- 

 quently spoken of as tetanizing currents, and the procedure as tetanization 

 or Faradization. These currents also increase in strength as the secondary 

 approaches the primary. 



Helmholtz's Modification of the Inductorium. With a view of equalizing 

 the strengths of the induced currents, Helmholtz suggested a device the adoption of 

 which accomplishes this to a certain extent. It consists (Fig. 342) in connecting with a 

 wire binding posts P' and S", and in providing binding post P" with an adjustable screw 

 which can be raised until the spring comes in contact with it, when the hammer is 



drawn down by the electromagnet B'. This 

 latter arrangement is practically a short- 

 circ\iiting key by which a portion of the cur- 

 rent is returned to the cell without ever enter- 

 ing the primary coil. The same arrange- 

 ment, though differently lettered, is shown in 

 Fig. 343. By the use of the entire device 

 the changes in the primary coil are made 

 not by making and breaking the primary 

 current, but by alternately long- and short- 

 circuiting the current. "When the short- 

 circuiting key is opened, the full volume of 

 the primary current flows through the pri- 

 mary coil. When the short-circuiting key 

 is closed, most of the current fails to enter 

 the coil, taking the easier path through 

 the key. Some of the current, however,, 

 always flows through the coil and is never 

 diverted. The cycle of changes in the 

 electric condition of the primary coil is 

 thus altered for two reasons: 



" First, we no longer have an alternation 

 between a full primary current and none 

 at all rather an alternation between a full 

 primary current and a weaker one. The 

 difference in the phases is thus lessened, 

 the extent of the change on making and 

 breaking is lessened, and correspondingly 

 the efficiency of the make and break cur- 

 rents induced in the secondary coil is 

 slightly decreased. 



"Second, on making the primary current, as in the ordinary coil, the sudden 

 appearance of the primary current is antagonized by the opposing make extra current , 

 with the result that the make induced current is still further reduced; while on break- 

 ing the current the break extra current can now flow through the primary coil across 

 the short-circuiting key. This current, trailing behind the disappearing primary 

 current in the same direction, produces the same effect as if the primary current itself 

 were to disappear slowly. As a result the disappearance of the primary current loses 

 its former efficiency as an inducer of secondary currents, and the break induction 

 current is reduced to about the efficiency of the make. 



"This so-called 'equalizing' of the make and break induced currents is never 

 perfect, if for no other reason, because the make extra current must take the long 

 circuit through the battery, while the break extra current has an easier path through 

 the short-circuiting key, and is thus greater than the make extra current." (C. C. 

 Stewart.) 



FIG. 343. HELMHOLTZ'S MODIFICA- 

 TION OF NEEF'S HAMMER. As 

 long as c is not in contact with d, 

 g h remains magnetic; thus c is 

 attracted to d and a secondary 

 circuit, a, b, c, d, e, is formed; c 

 then springs back again, and 

 thus the process goes on. A new 

 wire is introduced to connect a 

 with /. K. Battery. 



