Contributions to Physiologic<al Technique 361 



any suitable material can be fixed. Fine straws serve admirably, but I 

 prefer myself to use a strip of thin vshcet-magnalium/ 15-2 mm. wide 

 sharpened and bent to form a writing-point, and stiffened by having a line 

 drawn down its middle by means of a blunt metal point and straight-edge. 

 Such a magnalium lever can also be stiff'ened by making its transverse 

 section curved in outline by drawing it through a short piece of glass 

 tubing constricted at one end in the blowpipe-flame. If the fixed end be 

 suitably tapered no special aid to fixation is necessary for ordinary work, 

 but if desired a tiny wood or magnalium wedge can be employed. 



The bilateral symmetry of the apparatus both in tiie transverse and 

 longitudinal directions permits the writing-lever being fixed so as to point 

 backwards if required on either side. This conveniently avoids the 

 necessity of ever having the direction of the writing-lever against that of 

 the motion of the recording surface. The necessity of the writing-lever 

 projecting somewhat towards one side of the apparatus for application 

 to the recording surface, is better met by a torsion to the required 

 degree of the by no means fragile suspension by means of the fingers, than 

 by too much flexion of the writing-lever. Any flexion it may be desired 

 to give to it is better done by a sharp angle than a gradual curve. 



(6) Adjustability of the degree of pressure of the writing-point on 

 the recording surface is given by means of a strong flat spring suspension, 

 the tension of which is regulated by a screw. The latter is, however, 

 placed longitudinally instead of, as usual, transversely. Its ht^ad remains 

 therefore always conveniently accessible whatever 'the position of the 

 apparatus. 



As would be anticipated from its mode of cf)nstruction, iIh; iiiol)ility of 

 the signal is very considerable. It will respond easily to 100 interruptions 

 per second. Beyond this I have not yet tested it. The latei>cy of the 

 signal to the rupture of an electric circuit is of the same order as that of 

 the Pfeil and Deprez signals (0'5-la-), and is very constant under identical 

 conditions. 



Specimens of records by the double-signal are given in fig. 2. One 

 electromagnet was connected up with the Brodie-Palmer clock writing 

 seconds, and the other with the primary circuit of a Du Bois coil. The 

 speed of the recording surface was different in each tracing. Records of 

 make and break and of " tetanisation " are shown. The rate of interruption 

 during the latter is well shown in the lowest tracing (52 per second). 



The double signal was shown, in a form differing only in its 

 details, at the meeting of the Physiological Society on 1st March 

 1902. It is made by Mr John Sinclair, Physiological Labo- 

 ratory, University of London. I may add here that I have 

 applied the same principle of the electromagnetic shifting of the 



' Recording levers of magnalium were employed by Dr O. Rosenheim and myself for 

 heart-work in 1903. The much greater rigidity of this alloy of magnesium and aluminium, 

 with its actually less den.sity, makes it much more suitable for the purpose than aluminium. 



