for inequality of capillary attraction, as explained above), to represent the entire difference in pressure on the surface of the oil in the two drums, due to the draft of the chimney; that is, a certain known height of column, filled, in one tube with a mixture of alcohol and water of specific gravity 0.926, with the flue pressure resting on its surface, is just balanced by an equal height filled with olive oil of specific gravity 0.916, with the pressure of the atmosphere resting on its surface. The differential column, therefore, represents a water column one-hundredth part as high, or a column of mercury 1360 part as high. A draft which would be measured by 0.01 inch of mercury, or by 0.136 inch of water, would, on this anemometer, be measured by 13.5 inches of differential column. It is therefore a hundred times as sensitive as a water column and more than 1,300 times as sensitive as a mercury column. If too sensitive, so that the required range would exceed the limits of the instrument its sensitiveness can be reduced to any desired extent by a larger admixture of water, or by the use of pure water, as described by Welsbach, in which latter case the difference of specific gravity will be 1.000 0.916 = 0.084, and the sensitiveness 11.9 times as great as that of water alone and 160 times as great as that of a mercury column." use. = Prof. Carpenter:-There are some objections to this gauge in You have to know very accurately the relative specific gravity of the two liquids used. While it gives sufficiently accurate results for measurements such as Mr. Cary has spoken of, if very close measurements are desired, it will hardly compete with the instrument Prof. Kinealy has shown here. Mr. Cary:--I quite agree with Prof. Carpenter. I much prefer the other instrument. It is much simpler to use. In making a boiler test I have my two bottles with me, one of colored alcohol and the other oil, besides the glass draft gauge, and something is very apt to break, and the pouring in of the two liquids and pouring them out into the separate bottles afterwards, though it can be done easily, is a nuisance, and it is not got at as easily as this. But I have found it very satisfactory. The instrument itself I got directly from Mr. Barrus, and he calculated it very carefully from a regular U tube. It is calibrated under ordinary temperatures in connection with the U tubes. By taking a series of tubes you get a multiple effect carefully calibrated, and then the multiplication is noted on the back of the gauge, so that for each one-tenth of an inch on the scale you merely multiply it by this factor and you get the results directly. The other instrument, though, I regard as very much simpler, and one which I would prefer to use. Prof. Carpenter:-This (exhibiting a gauge) is the draft gauge which was unfortunately broken this morning and which I have had repaired. I will explain the principle of it, then show how it can be used. In one form, Fig. 27, it is an ordinary draft or pressure gauge It has an advantage over most gauges in the fact that one single reading gives the result. It may be described as follows: The accompanying cut, Fig. 27, shows the gauge one-half size. The form of the draught gauge is U-shaped and consists of the m F following parts: The U glass tube B D, air cock C for hose connection, cup A, guides G and F, vertical scale I, which is divided into inch, half, quarter and one-twentieth of an inch, screw K for adjusting index pointer O and having twenty threads to the inch; the index wheel is divided into twenty-five parts, and the whole combination is raised and lowered by screw E; S serves as a protection for glass tube and bearing for E. Since the screw K has twenty threads to an inch, and the scale is divided into of one inch, then for one turn of K it moves index O one division, and since the index wheel has twenty-five divisions, each division is then equal to 5 of 6=500.002. For example if second division to the right of zero on the index wheel were placed on R, it would then be read 1++ (500X2)=1+0.1+.004 1.104; i. e., each small division on scale I is equal to 0.5, and on index wheel equal to .002. = The method of operation as a draft gauge is as follows: The glass tube is filled with water to about one-third its height, one end of a flexible rubber tube is then connected to cock C, and the other end is inserted to the chimney flue. B is then open to the atmosphere and D is in communication with chimney flue. There will now be a difference of water level in tubes B D. Adjust m so that it will be tangent to bottom of meniscus by screw E, and n by means of screw K. If it is desirable, cock C can be closed and the difference of water level maintained and taken to some convenience place to read. Fig. 27. This gauge can be read very closely, and the difference of height in both gauges is given by a single reading and without reduction. In using the draft gauge as an anemometer, for measurements of the velocity of the air, the two branches of the draft guage are connected respectively to two tubes, one of which, as D in Fig. 28, is straight and has an open end which is placed perpendicular to the axis of the pipe in which the air is passing whose veloctiy is to be measured. The other tube is bent so that the opening directly faces the current of moving air. Both these tubes for convenience are inserted in a larger tube, as shown by the section on A B. The current of air blowing in the mouth of the tube P depresses the water in the draft gauge, say to M', and from one branch of the U tube and raises it a corresponding amount in the other. The pressure in the opening D is communicated by the connecting tube to the other branch of the gauge and since this pressure acts the same in both tubes, the effect on the draft gauge is the same as though no pressure existed, the difference in the height in the two branches being due entirely to the velocity of the moving air. This device is an old one and is a modification of an instrument known as Pitot's tube. In the use of this tube it is necessary that the opening P shall squarely face the current of moving air and that the opening D shall be in a plane at right angles. There being no motion in the tubes P and D communicating pressure to the gauge, there is no correction for friction, and the velocity of the air will be that given by the formula v=k √2gh. If water be used in the draft gauge and h be taken in inches, the value of v or the velocity in feet per seconds is v = 70 Vh when the air is at a temperature of 120 degrees. If the air is at other temperatures this result must be multiplied by the following factors: The diagram, Fig. 29, presents a graphical method of obtaining the results respectively on the left and right. Thus if the scale read 1.1 1.2 1.3 1.4 1.5 1.6 1.7 1.8 1.9 2 Fig. 29. has four scales, there being two on each side. If the scale reads. between one and ten inches of water or between one and ten hundredths of an inch of water the lower scales are to be used, reading the results respectively on the left and right. Thus if th scale reading is 3.2 inches, the corresponding velocity is 126 feet per second; if the scale reading is .32 of an inch the corresponding velocity is 12.6 feet per second. The scales in the upper part of the diagram are to be used in a similar manner when the readings are between one-tenth and one inch and also between one-thousandth and tenthousandths of an inch. Thus, if the reading in the water gauge is 0.36 inches the velocity is 42 feet per second; if the reading were .0036, the velocity would be 4.2 feet per second. The description of this instrument is a digression from the subject of the paper and a longer description would be inappropriate in this place. A more complete description of the method of using this as a substitute for an anemometer will be taken up more fully at a later meeting. |