« PreviousContinue »
creation of much more powerful voltage If the impact of an ether wave be then in order to bring about the required received, it will affect the filings suffiresults.
ciently to render them more
or less The third and last question arises: parallel, break down the resistance of the How can these ether“ vibrations be irregular air spaces, and operate the bell detected? The original method, and the or key, thus giving a dot or dash signal one in use at the present time by most as required. A second small battery is
arranged so as to de-cohere automatically the filings after each dot or dash.
Comparatively recently, Marconi invented what has been called his magnetic detector. This acts on a different principle. A coil of soft iron wire is made by mechanical means to pass near the poles of a fixed magnet. This has the effect of magnetizing and demagnetizing the wire while passing the different magnetic poles. The demagnetization, however, is incomplete, a condition known as hysteresis remaining. This completely
and rapidly disappears when subject to INSTRUMENT ROOM, HOLYHEAD STATION. the impact of a Marconi wave, resulting
in a sound or click readily detected by systems of wireless telegraphy, is by an ordinary telephone-receiver attachmeans of a coherer. A brief description ment. This method is now adopted for of the principle underlying this will help all long-distance work. It is very simple us to understand its working. If we in character, and admits of much greater were to take some soft iron filings and place them in a vial around which was wound an ordinary copper wire that could be connected in the circuit of an electric battery, we should find that the iron filings would be irregularly distributed until we passed a small electric current through the wire surrounding the vial. Then they would, through magnetic force, all arrange themselves in a parallel direction and would remain so until the current was cut off and the vial mechanically disturbed. A similar effect, only much lighter, takes place from the impact of a Marconi ether wave. It has also been found that a mechanical mixture of silver and nickel filings with mercury is much more sensitive to the impact of a wave than iron, and these are now used in a small glass tube. Taken together this is technically known as the “coherer.”
This coherer is then placed in the MARCONI'S WIRELESS CAR CHARIOT. circuit of an ordinary electric door-bell circuit, or in that of a Morse instrument, rapidity in the reception of messages and its connecting wires penetrate the than the use of the coherer. filings just far enough so that the current One thing further must have our conof electricity cannot operate the Bell or sideration. We have all heard or read Morse instrument owing to the resistance of tuning, so that messages could be of the irregular air spaces in the filings. received only by certain instruments,
which are exactly tuned or syntonized to messages sent from the same or different the transmitter, this being to a certain places, as was admirably demonstrated in degree necessary in order to protect the England before Professor Fleming, when secrecy of the message. We know what he succeeded in taking messages in Engtuning is in connection with vibrations lish and French simultaneously on superin air. If we take two tuning forks of imposed instruments. precisely the same key, placing them The above narration is intended not as some distance apart in a room, then start a scientific treatise, but a simple stateone vibrating, the other will take up the ment of the principles involved. Marconi, same tone, continuing it for some time. though young, has already accomplished If, however, the instruments are of dif much. The bridging of the Atlantic by ferent keys, or, in other words, are not etheric signals, though Utopian but a tuned to each other, the second one will short time ago, has been successfully remain unaffected.
accomplished; and a new force, hitherto A similar condition of things exists in unknown, has sprung into existence. It connection with ether vibrations. Owing will find its place in the onward march however, to their enormous frequency, of civilization and commerce, as have the the tuning possibilities are very much telegraph, the cable, and the telephone ; greater. Marconi has, by a simple con reaping its share of commercial reward as trivance, been able to accomplish this. did they, and all through the remarkable By this tuning arrangement, he can in prescience and genius of one man, Marthe same room receive, without confusing coni.
The only buildings in the world which The latest type of British submarine are earthquake-proof are the Japanese boat, built by Messrs. Vickers Sons pagodas. There are many which are & Maxim, has attracted much atten700 or 800 years old and as solid as tion in naval circles. It has a speed unwhen first built. The reason lies in their der water of between nine and ten knots. construction. A pagoda is practically a It is a modified development of the Holframework of heavy timbers which starts land type, the length of the new boat befrom a wide base and is in itself a sub ing 150 feet and 120 feet in the original. stantial structure, but is rendered still The radius of action in the new type is more stable by a peculiar device. Inside 500 miles; and in the Holland, about 300 the framework and suspended from the miles, apex is a long, heavy beam of timber, two The weight is too great to permit the feet thick or more. This hangs from boats to be carried on shipboard. In one end of the four sides. Four more practice evolutions, the submergence heavy timbers, and, if the pagoda be usually lasts three hours, but the quarvery lofty, still more timbers are added
ters are very cramped and the crew is to these. The whole forms an enormous obliged to keep fixed stations, as the dispendulum, which reaches within six placement of the center of gravity might inches of the ground. When the shock cause the boat to take a disastrously deep of an earthquake rocks the pagoda, the plunge. It has been suggested that three pendulum swings in unison and keeps sets of these boats should be provided, the center of gravity always at the base each of which would have three days on of the framework. Consequently the duty and six days off, in order to preequilibrium is never disturbed.
serve the health of the crew.
HE rapid increase in the number out of each pound of live steam furnished
of isolated lighting plants in to the engine, there will be 960 X.8=768 vests with a greater than usual heat units available for heating purposes.
importance the question of the It requires practically 210-40=170 heat proper utilization of exhaust steam. In units to raise one pound of feed-water general, we may say that it is a matter of from 40° to 210°; and 178=t, or economy to use this for heating pur between one-fifth and one-fourth. Takposes, although various factors must be ing the larger figure, we have left, for considered in each case to determine to heating or other purposes, .75X.8=.6 of what extent this is true. The more im the entire quantity of steam supplied to portant considerations bearing upon the the engine. The principal objection to subject are the relative quantities of the use of exhaust steam for heating has steam required for power and for heat been the higher back pressure required ing, the length of the heating season, the
B type of engine used, the pressure carried, and, finally, the question whether the plant under consideration is entirely new F or whether, on the other hand, it involves the adapting of an old heating system to a new plant. The first use to be made of the exhaust
E steam should be the heating of the feedwater, as this effects a constant saving both summer and winter, and it can be done without materially increasing the on the engine. The loss in power of an back pressure on the engine. In ordi engine from back pressure is nearly pronary practice, only about one-fifth to portional to the ratio of the back pressure one-fourth of the exhaust steam can be to the mean effective pressure.
This is used in heating the feed-water from a shown graphically in Fig. 1. temperature of 40° up to 210°. This is Let the work done by one stroke of easily shown as follows:
the engine be represented by the area We may assume, under ordinary con A B C D E. If the rectangle F GDE ditions, that about 80 per cent of the be drawn, having an equal length and steam supplied to an engine is discharged the same area, then the mean effective from the engine in the form of steam at pressure will be represented by the height low pressure, the remaining 20 per cent EF. Let E D be the normal back presbeing lost through condensation, etc. sure line ; then if it be raised to the posiThe latent heat of a pound of steam at tion I J, the resulting lost work will be 2 pounds gage pressure is 960; therefore, given by the area I J D E. As the
rectangles FGDE and I JD E have the same length, their areas as their heights, which represent the mean effective and back pressures respectively. From this it is evident that any increase in the back pressure cuts down the power in proportion to the ratio of the increase in back pressure to the mean effective pressure.
Example. The M. E. P. of an engine is 40 pounds and the back pressure is raised 5 pounds; what will be the loss in power?
Five-fortieths=18, or 12.5 per cent, loss in power.
There are two ways of offsetting this loss. One is by raising the initial or boiler pressure, and the other is by increasing the cut-off of the engine. The first method is illustrated graphically in Fig. 2.
Let A B C D E be the indicator diagram of an engine exhausting against atmospheric pressure. If the back pressure line is raised to the position I J, the lost work per stroke will be represented by the area I J DE; and, to offset this, the line of initial pressure must be raised to such a height that the area A K L MC B will equal the area I J D E. The required increase in boiler pressure for offsetting the effect of a higher back pressure may be found approximately by the use of the following table :
cent. It is desired to use the exhaust steam for heating, at a pressure of 5 pounds per square inch. How much must the boiler pressure be raised in order that the engine may have the same power at the same cut-off ?
It is evident that if the power of the engine is to be kept the same, the mean effective pressure must remain constant. The normal back pressure on an engine exhausting into the atmosphere is about 2 pounds gage. Referring to the above table, we find the ratio for 14 cut-off and 5 per cent clearance to be .62, from which the mean pressure in the above case is (80+15) X.62=50 pounds, and the mean effective pressure is 59-17=42 pounds.
MEAN PRESSURE RATIOS
If the back pressure is to be raised to 5 pounds gage, or 20 pounds absolute, the mean pressure must also be increased 3 pounds in order to keep the mean effective pressure the same. This calls for a mean pressure of 62 pounds absolute; and, dividing this by .62 (the ratio for 14 cut-off and 5 per cent clearance), we have an initial pressure of 83 pounds called for, instead of 95 as in the first case.
This shows that under the conditions of the problem an increase of 3 pounds in back pressure requires a corresponding rise of 5 pounds in the boiler pressure to keep the power of the engine the same with a constant cut-off.
The effect of the second method (that is, increasing the cut-off) is shown in Fig. 3, in which the added area B KLC must equal I JD E. The necessary increase in the cut-off for any given case can also be found by the use of the table already given.
E.rample. An engine cutting off at 1/5 stroke is supplied with steam at pounds gage pressure. Clearance of the
PER CENT CLEARANCE Cut-off
0 1.75 3 5 7 9 1/10 .33 .35 .36 .39 .41 .43 1/6 .40 .41
.47 1/6 .46 .48 .49 .50
.53 15 .52 .53 .54 .55 .57 .58 1/4
.59 .60 .61 .62 .63 .64 316 .67 .68 .68 .69 .70 1/3 .69 .70 .71
.72 .73 3,8 .74
.75 .75 .76 2 5 .76
.78 .78 79 7/16 .79 .80 .80
.81 .82 To find the mean effective pressure exerted throughout the stroke, multiply the absolute initial pressure (above a vacuum) by the number opposite the given cut-off and in the column corresponding to the percentage of clearance, and deduct the absolute back pressure.
E.xample. An engine cutting off at 14 stroke is supplied with steam at 80 pounds gage pressure, and exhausts into the atmosphere. The clearance is 5 per