THE GRAPHIC METHOD 



13 



D. REGISTRATION BY PHOTOGRAPHY 



Even the most delicately constructed writing lever has some weight, and 

 hence, because of its inertia, may give an incorrect form to the curve. The 

 ideal recorder would be entirely without mass. We have such a recorder in 







FIG. 12. Capillary electrometer, after Love"n. The instrument is mounted so that it can be 

 placed on the stage of a compound microscope. 



a beam of light, provided the experiment can be so arranged that the move- 

 ment to be recorded is transmitted directly to a small mirror which reflects 

 the beam of light, and the reflected beam can then be made to fall on a 

 moving surface which is sensitive to light. 



But the pliot o graphic method is of much greater importance for recording 

 movements which cannot be recorded in any other way. 



This is the case, for example, with the excursions of the capillary electrom- 

 eter. This instrument (Fig. 12) consists of a fine capillary tube partially 

 filled with mercury and dipping into a 

 dilute solution of sulphuric acid so 

 that the mercury comes in contact 

 with the acid in the tube. When elec- 

 tricity from any source is led into the 

 instrument by connecting one pole 

 with the Hg and the other with the 

 H,SO 4 , the mercury meniscus in the 

 capillary tube will move in the direc- 

 tion of the current. Such movements 

 can be magnified by a microscope and 

 be recorded on a moving photographic 

 plate. Since many forms of activity 

 in the animal body are accompanied 

 by electrical changes of potential 





FIG. 13. Action currents of the dog's heart as 

 recorded by photographing the excursion^ 

 of the mercury column in the capillary 

 electrometer, after v. Kries. Electrical 

 connection was made with the base and 

 apex of the heart. First phase : base nega- 

 tive to the apex. Second phase: apex 

 negative to the base. The upper line rep- 

 resents the time in fifths of a second. To 

 be read from left to right. 



which cannot be demonstrated in any 

 other way than by a very sensitive 

 electrometer, this mode of registra- 

 tion is very valuable for the study of such phenomena. Fig. 13 represents the 

 photographic curve of the electrical variations (action currents) appearing during 

 the cycle of events in the dog's heart. 



