284 Lieut.-Colonel C. B. Heald and Major W. S. Tucker. 



1. The Microphone. 



The hot-wire instrument is specially capable of dealing with low-frequency 

 vibrations such as those imparted to the human body by the heart. It 

 consists essentially of two iron drums, connected together by a short piece of 

 rubber tubing. 



One of the drums is fixed to the ceiling of the room in which the work is 

 done. It consists of a cylinder, the side of which is about half its diameter. 

 The platform on which the patient is standing is supported by a hook from 

 a diaphragm which forms the circular end of the drum. When, therefore, the 

 heart-beats cause motion of the body, the diaphragm responds, thereby altering 

 the pressure within the drum in a corresponding way. 



A side tube, let into the wall of the drum, permits the passage of air which 

 is set in- motion as a result of these changes. This tube is connected by the 

 rubber tube above mentioned to a second cylinder of about twice the capacity, 

 and having conical ends, in one of which is a fitting containing the hot-wire 

 microphone grid. The air blasts, transmitted along the rubber tube from the 

 first drum, are passed into the second drum through a tube opening, and are 

 projected past the hot-wire grid, thereby cooling the grid and diminishing to 

 a corresponding extent its electrical resistance. 



Eecords of apex beat or carotid involved the use of a second microphone, 

 simply consisting of an open cup whose rim is of ebonite and whose base 

 contains the microphone fitting with its grid, and the image of the " string " 

 recording these effects is thrown on the same recording strip as the recoil 

 curve by means of a right-angle prism. The cup is pressed with its rim 

 against the chest or neck, and the pulse changes are indicated by air blasts 

 passing the grid into the open air (fig. 1). 



The breathing microphone (fig. 1) consists of a single strand of hot-wire, 

 mounted on an appropriate fitting attached to a stem resembling that of 

 an ordinary tobacco pipe, but with a much wider air channel. The 

 mouthpiece is held in the mouth, and the temperature of the grid is 

 simply varied by that of the air or of the breath which passes inwards 

 or outwards. 



Periodicity of the breathing is thus indicated. A very small electric 

 current is used, just sufficient to indicate change in resistance on the 

 galvanometer. 



2. The Galvanometer and Timing Device. 



The galvanometer employed is of the Einthoven type. Body movements 

 are measured by deflections of a very fine wire mounted between the poles 

 of a strong electromagnet. The instrument employed is the Souttar galvano- 



