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TEXT-BOOK OF PHYSIOLOGY. 



coils, the bars are again magnetized, the hammer is drawn down, to be 

 followed by a second break of the circuit. 



The number of times the circuit is thus made and broken per second will 

 vary with the length of the spring. 



As each interruption of the primary circuit develops an induced current, 

 it follows that the latter must succeed each other with a frequency corres- 

 ponding with the frequency of the former. If while the primary circuit 

 is thus being interrupted the wires of the secondary coil be placed in contact 

 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 frequently spoken of as 

 tetanizing currents, and the procedure as 

 tetanization or Faradization. These cur- 

 rents 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 con- 

 sists (Fig. 362) 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 arrange- 

 ment is practically a short-circuiting key by which a 

 portion of the current is returned to the cell without 

 ever entering the primary coil. The same arrange- 

 ment, though differently lettered, is shown in Fig. 363. 

 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 



FIG. 363. HELMHOLTZ'S MODIFI- 

 CATION OF NEEF'S HAMMER. As 

 long as c is not in contact with d, 

 g h remains magnetic; thus c is at- 

 tracted 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. 



through the primary 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 currents 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 breaking 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.) 



THE GRAPHIC METHOD. 



The term graphic is applied to a method by which curves or tracings are 

 obtained which represent the extent, duration, and time relations of the 



