PHYSIOLOGIC APPARATUS 



775 



of sensitiveness, two strong magnets are employed, across the field of which 

 runs a very fine conductor usually made by drawing out molten quartz, by 

 means of a bow and arrow, to a thread much thinner than the smallest possi- 

 ble metal wire, and then silvering the filament to enable it to conduct elec- 

 tricity. When the filament is securely fastened at its extremities, it will be 

 found that it will deviate to the right or the left according to the direction of 

 the current which enters it. 



By reason of the small size of the filament employed, it must be strongly 

 illuminated, and the shadow of the thread must be magnified by an optical 

 device and then projected across a narrow slit on a sensitive surface behind 

 it. As the shadow of the thread moves from side to side at the same time 

 that the sensitive surface moves in a direction at right angles to it, a graphic 



FIG. 351. THE CAMBRIDGE ELECTRO-CARDIOGRAPH OR STRING GALVANOMETER. 



record will be obtained in which the abscissa will indicate time and the ordi- 

 nates, the displacement of the thread or the strength of the current passing 

 through it. A typical normal record or electro-cardiogram (Fig. 130, page 

 292) shows three upward directed waves separated by two smaller downward 

 directed waves and these are commonly designated, following Einthoven, 

 by the letters P, Q, R, S, T in order. In the conduct of an experiment a 

 patient's extremities are connected with some suitable low-resistance elec- 

 trodes. Under such circumstances a part of the heart's action currents may 

 be led off to the filament of the galvanometer. If the circuit is completed 

 from the right arm and the left arm, the arrangement is called lead I ; if from 

 the right and left leg lead II; if from the left arm and the left leg, lead III. 

 The apparatus is customarily arranged so that the current produced by 



