290 COHN— STUDY OF THE [April 23. 



ment built on similar lines for use as an ocean telegraph recorder 

 had already been devised by Ader, but of this he was unaware at the 

 time. It depends on the principle that a conductor suspended in a 

 magnetic field is deflected at right angles to the lines of force when 

 a current passes through it. The conductor chosen is usually a 

 silvered quartz or platinum thread, 87 to 100 mm. long, 3 to 6 micra 

 thick, having a resistance of 3,000 to 6,000 ohms It is suspended 

 vertically between the poles of two powerful electro-magnets (Fig. 

 2). The thread is illuminated by the rays of an arc light which are 

 focused on it by a system of lenses and a substage condensor. To 

 accommodate this condensor the pole of one electro-magnet is bored. 

 The motions of the string are magnified and projected on a record- 

 ing photographic surface by a microscope held in a similar bore in 

 the pole of the other electro-magnet. The degree of magnification 

 may be varied according to the needs of the investigator, but, in 

 order to maintain a degree of uniformity in the appearance and in 

 the electrical value of curves obtained in different laboratories, cer- 

 tain arrangements have become conventional. These include the 

 strength of the magnetic field, the tension of the string and its de- 

 flection time. The strength of the field depends, of course, on its 

 construction. The tension of the string is adjusted in an appropriate 

 manner so that a current having the value of one millivolt, when 

 allowed to pass through it, causes a deflection of i cm. It has been 

 found desirable to obtain a deflection of this extent within a definite 

 length of time, usually 0.02 seconds or less. When the deflection 

 time is longer, certain waves in the electrocardiogram tend to dis- 

 appear. 



But in order to obtain an electrocardiogram, more is necessary 

 than to include an individual in the string circuit. For although the 

 usual cardiac action current does not deflect the string, when it is at 

 the prescribed tension, beyond the optical field, the skin or constant 

 current, which is also continuously discharged from the body, and 

 which is composed of the summed discharges from the other elec- 

 trically active tissues of the organism, may do so, and it is usually 

 sufficiently great to deflect the string far beyond the field of the 

 microscope. This current does not show the rapid changes of potential 

 difiference that the action current from the heart does. On the 



