640 



PRINCIPLES OF GENERAL PHYSIOLOGY 



i iiiiiiii liiiiU.i. 





Company may be consulted. In general, if the deflection required is to be 

 as large as possible, or a very small electrical current is to be detected, 



the Paschen form of the Kelvin 

 instrument is the best. If the 

 time relations of a complex change 

 are to be ascertained by photo- 

 graphic record, the " string " gal- 

 vanometer of Einthoven is to be 

 used. This latter form is that 

 with which most work is now 

 done. Fig. 200 represents the 

 instrument as made by the Cam- 

 bridge Instrument Company. A 

 camera with moving plate or paper 

 is used with it, and an arc lamp 

 for illumination. The shadow of 

 the string is photographed. A 

 diagram of the mechanism is given 

 in Fig. 201. 



In the Oscillograph of Duddell, 

 the movement of the system follows 

 changes of current rapidly by the 

 fact that its own vibration period 

 is enormously high compared with 

 the rate of change of the current 

 to be investigated. This vibration 

 rate may amount to 10,000 per 

 second. The same principle is 

 used for the correct registration 

 of the pressure changes in the 

 heart and the blood vessels. The 

 oscillograph is practically a moving 

 coil galvanometer, with the coil 

 reduced to a thin loop of stretched 

 phosphor-bronze. It is used chiefly 

 for the investigation of the wave 

 form of alternating currents. 



Electrometers. Although there 

 are several forms of instruments 

 used in physical work for measure- 

 ment of potential, the capillary 

 electrometer of Lippmann is practi- 

 cally the only one used in physi- 

 ology. It consists of a slightly 

 conical capillary tube, containing 

 mercury, whose point is immersed 

 in 20 per cent, sulphuric acid. 

 The mercury is forced into the 

 capillary tube by pressure until 

 the meniscus of its contact with 

 the acid is at a convenient place. 

 One electrode is connected with 

 the mercury in the tube, the other 

 with a mass of mercury also in con- 

 tact with the acid. Fig. 202 shows 

 one pattern of the instrument. 



When connected to two points at different potentials, so that that which is positive 

 is in connection with the mercury in the capillary, the surface tension at the contact 

 with the sulphuric acid is diminished, and the mercury is forced by the pressure 



' ' i" *v*rl. J i. Ur+rtrhtrtr+rir 



r 



Fir;. 199. Electro-cardiograms of the human heart, 

 taken with different tensions of the fibre of 

 the string galvanometer. The tensions di- 

 minish in the series from above downwards. 

 Led off from right arm and left leg. The curve 

 to the right of each figure shows the time 

 taken to attain full deflection when one milli- 

 volt is applied. This time increases as the 

 tension of the string diminishes, although the 

 movement is aperiodic in all the cases given. 

 It will be noticed that the rapid changes in 

 R and S are not correctly followed with low 

 tension, but that the slower rate of change in 

 T is equally well given in all. 



(Lewis, 1913, 1, p. 10.) 



