10 GENERAL PHYSIOLOGICAL METHOD 



membrane (not shown) upon which is fastened by means of cement a small disk 

 supporting a rod which carries a magnifying lever. The height of the writing 

 point, which is fastened to the lever, may be varied by means of the thumb- 

 screw at the top. Errors due to inertia of the blood-column itself may be still 

 further diminished by damping* its movements with the stopcock between the 

 chamber and the artery. A thumbscrew at the right enables one to graduate 

 this resistance as required. ED.] 



In case it is desired to continue the record of the blood pressure or of 

 other physiological movements uninterruptedly for a long time, a kymograph 

 carrying two drums, placed at some distance from each other and so arranged 

 that the smoked paper extends around both, may be employed, and a longer 

 recording surface is thus obtained. A still longer record can be made in ink 

 on a white surface by means of a kymograph carrying " endless " paper (Fig. 6). 

 By means of a " pen " of suitable construction, a record can be continued for 

 days, the paper being unrolled from one spool and rolled onto another at the 

 proper rate of speed, after the ink has had time to dry. 



B. TIME RECORDERS 



It is often necessary to know the speed at which the revolving surface 

 passes the writing point. This may be determined roughly by regulating the 

 revolutions of the drum to a certain uniform speed; but if the exact temporal 



m 



FIG. 7. An electric signal used as a time marker, after Deprez. 



relations of the events which are being recorded must be known, it is necessary 

 to employ a time marker. This instrument commonly takes the form of an 

 electric signal. 



A convenient form is that represented in Fig. 7. An electro-magnet (m) 

 bears on its armature a recording lever (s) which can be arranged so as to write 

 on a smoked surface. The movements of the armature, and hence those of the 

 lever, are determined by making and breaking the current to the magnet at 

 regular intervals of time. If it is desired to mark seconds, as in Fig. 4, a clock 

 beating seconds may be so arranged as to make and break the current. If small 

 fractions of a second are required as in Fig. 8, a tuning fork vibrating the 

 desired number of times per second may be made to dip a platinum wire in and 

 out of mercury with each vibration and so interrupt the current. 



The tuning fork itself may also be employed as the time marker by attach- 

 ing a very light writing point to one of its prongs and arranging this so that 

 it will make a light tracing on the recording surface while the tuning fork is 

 in vibration. 



It will be apparent: (1) that the time interval to be employed must be 

 adapted to the speed of the drum and this in turn to the rapidity of the events 



