An Electro-Magnetic Sphygmograph ∗

Abstract

While there are several more or less satisfactory mechanical devices adapted for the transcription of pulse pressure waves from superficial arteries (carotid,† radial,‡ digital§), excellent results are not always easily attained. The adaptation of the electro-magnetic principle here described, while simple, seems to present a sturdy as well as a foolproof device. The chief assemblage includes a 200 ohm, single Button Model W carbon granule microphonel¶of excellent sensitivity which, by the adaptation of a short rod fixed upon its diaphragm, is held or strapped over a suitable throbbing artery serving in place of a dispatching tambour. It is to be noted here that its function is the reception not of sounds but of the pulse pressure waves as such, and it acts as a variable resistance to the passage of an electric current. Thus the resistance is caused to vary inversely with the differential pressure throbs of the artery. At the recording end of the electrical circuit in this assemblage, a permanent cylindrical alnico magnet, 3/8″ in diameter and 5″ long is fixed in an upright position, supported upon an adjustable iron stand. About the magnet is placed an ordinary 6″ test tube with bottom removed, upon which is wound about 200 turns of 28 gauge cotton covered magnet wire of approximately 1.7 ohm total resistance; each end of the wire being coiled somewhat tightly to serve as an elastic mounting of the tube and its coil. The ends of the wire constituting the coil then serve as supports as well as terminals through which the current passes. One pole of the D.C. battery is connected to one end of the wire mounting; the other battery pole is connected through the microphone to the other terminal.