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the indicator is actuated through two gear-wheels, which are in turn operated by a plunger which is seen to pass through and be attached to the bottom of the lowest chamber. It

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Fig. 17.-Diagrammatic representation of internal construction of a wellknown pocket sphygmomanometer. Note the two compression chambers, the single nipple and the arrangement whereby the compressing air is directed within, and expands the chambers.

will be seen also that these chambers communicate with each other and with the air in the body of the apparatus. The two attachments which serve as connections for the pump and the arm-band are seen to be interchangeable and to communicate with a closed space which surrounds, but which does not communicate with, the interior of the chambers. This is the vital point which assures accuracy in this instrument for, when air pressure is increased around the chambers, they tend to become collapsed and, driving the plunger upward, operate the indicator through the gears. The amount of movement depends upon the amount of pressure exerted upon the chambers, of which there are four.

Reference to Fig. 17 will show that the principle here is the reverse, and that the two chambers are designed to receive the increment of air, which, by expanding them, operates the plunger attached to their upper face which in turn actuates the indicator through a series of gears. In this instrument the over-expansion of the chambers is a constant possibility which, occurring even once, renders the apparatus permanently incorrect, while in the Faught the pressure can only collapse the chambers which, even when over-compressed, cannot more than force them into contact, and as this manipulation is applied to every Faught apparatus before it is graduated, the danger of inaccuracy is entirely removed. Another point in favor of this instrument is the employment of four chambers instead of two; this greatly reduces molecular motion and is another element in reducing the possibility of inaccuracy.

9. Faught pocket sphygmomanometer (Fig. 18) has a dial of white enamel, somewhat similar to that of a watch. The scale is graduated in millimeters of mercury, as determined by accurate calibration with a standard mercury column. The numerals are in red and black, to facilitate reading, and each individual graduation represents 2 mm., giving a working scale extending from zero to 300. No mathematical calculations are necessary to compute the pressure, which can be easily read directly from the dial.

In order to insure accurate and unvariable readings at all points on the scale, a factor of safety of 150 mm. has been provided, i.e., each Faught pocket sphygmomanometer before leaving the factory is tested up to 150 mm. above the

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Fig. 18.–Faught pocket sphygmomanometer attached to arm showing position of arm-band, dial attached to hook and arrangement of tube connections.

300 on the scale, or to 450, after which the readings must correspond with those of a standard mercury column, and the needle after this severe test must return immediately to zero. This shows clearly that, with ordinary use, it is practically impossible to distort the compression chambers of the instrument.

The accuracy of the so-called aneroid, or diaphragm type of sphygmomanometer has been questioned by some,

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but as already stated in the improved type of this instrument sudden variations are very unlikely to occur. The so-called “fatigue of metal” referred to by some authorities does not exist, and any error which manufacturers admit might develop in their instru

FAUGHT ments, must be due to some mechanical defect.

An additional member of the socalled aneroid group is the “clinical” pocket sphygmomanometer which has been placed on the market at the author's suggestion. This instrument is shown in Fig. 20. It is identical in construction to the pocket instrument differing only in

Fig. 19.-Faught pocket having a specially large dial (372 in.) apparatus dial in detail. and consequently, a more easily read scale, reading to 350 mm. Hg. The markings on this scale are sufficiently distinct to be easily read at a distance

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Fig. 20.—Clinical sphygmomanometer; side and front views; 42 actual size.

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of 10 to 15 ft., an advantage that will recommend itself to class teaching.

The instrument employs the flexible bandage-cuff or arm-band, (see Fig. 18), the inflatable portion of which

measures 5 X 9 in. A small metal pump with exhaust valve attached is supplied and these parts are all contained in a Morocco pocket

The needle of the in

dicator is extremely delicate BUILT LIKE A WATCH

and so sensitive that a diasFig. 21. — Faught pocket sphygmo- tolic reading can be made in manometer in case.

any case in which a mercury manometer would accomplish it.

10. Erlanger's Sphygmomanometer (Fig. 22).—This instrument in its improved form is apparently the most accurate yet devised for determining blood pressure, being based upon the same principle as the other instruments, but both the return of the pulse and the point of maximum pulsation are made clearly visible, thus almost entirely removing subjective errors.

The construction of this instrument is much more complicated though the only essential difference is the addition of an original recording device. The U-tube manometer connects with a four-way tube, of which one branch leads to the armlet, and another to a special stopcock. The vertical branch communicates with the interior of a rubber bulb, enclosed within a heavy glass bulb, which in turn, under certain conditions communicates freely with the atmosphere through another tube returning to the special

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