Light

Switch

Switch

Three-Way Switch FIG. 2.

Answer: Referring to sketch No. 1, the full lines represent the part which you show. The sketch has been completed with dotted lines indicating the proper method of wiring to obtain the desired result. One turn of the switch will throw off the light which is burning, and throw on the other, in every case. The wiring shown in sketch No. 2 is entirely correct for the three-way switch as it is used for the purpose of controlling a light from two separate points. The light can be turned on or off from either of the two three-way switches shown.

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St. Louis, U. S. A.

Probable M. E. P. of Steam Engines

Question: Please explain how to find the Mean Effective Pressure of a 6 x 12 steam engine making 250 revolutions per minute, the boiler pressure being 80 pounds by gauge.— F. J.

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CONSULTING DEPARTMENT-(Continued)

Answer: Your question cannot be given an absolute answer, for the reason that the M. E. P. depends on other factors than those which you have given. In other words, it would be necessary to know the cut-off, the back pressure, and type of engine. Making certain assumptions, we can give you an approximate answer. If the cut-off is 14, then the ratio of expansion is 4, neglecting clearance. Assuming the back pressure to be 3 pounds gauge, then the M. E. P. can be found by the formula

We have made no allowances for losses due to compression, clearance, condensation, wire drawing, etc. To allow for these in a plain slide-valve engine, it is customary to multiply this theoretical M. E. P. by the factor .8, which will give us for the probable M. E. P., 38.8 × x .8 ( 31.04) pounds.

In the formula used above, you will find the term "hyp. log. R." We are to substitute for this term the hyperbolic logarithm, or, as it is sometimes called, the Napierian logarithm of the factor R. These logarithms will be found in almost any engineer's handbook.

Exhausting Under Water

Question: Is it possible for two-cycle gas engines to exhaust under water without seriously reducing the H. P. of the engines?— B. O.

Answer: This is frequently done without loss of H. P. by extending the terminal under water, with a small vent above the water. The vent prevents the water from drawing back to the muffler when the motor stops.

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Mention The Technical World, Dec.

CONSULTING DEPARTMENT-(Continued) Current of Open and Closed Motor

Question: Upon the name plate of a G. E 500-volt direct-current compound-wound motor is the following:

"Open....10 H. P.....17 amperes.

Closed....5% H. P.....10 amperes."

Will you kindly explain the meaning of this?— H. C. J.

Answer: This doubtless refers to the condition of the machine running in the first place as an open motor, and in the second as an enclosed motor. It will be noticed that a current of 17 amperes is allowed in the first case, or when the machine is running as an open motor; and a current of only 10 amperes, when the machine is running as a closed motor. This discrepancy is, of course, due to the difference in ventilation, the open motor being much better ventilated than the enclosed.

Domestic Heating System

Question: I wish to put a heating system in my house. Kindly give me some rules by which 1 can determine the size of pipe necessary, the dimensions of radiators, etc.-H. B.

Answer: Your inquiry is entirely too indefinite, since there are so many variable factors connected with any heating system, and you have omitted to state the exact conditions. The principal points to be considered are:-First, the system itself, i. e., whether a furnace, steam, or hot water is to be used. Second, the location and exposure. Third, the material of which the building is constructed, and the purpose for which it is to be used. The following general rules can be used for rough calculation; but when it comes to actually laying out the plant. more exact rules which necessitate the knowledge of the conditions outlined above, should be used.

Using steam, one square foot of direct radiation will supply from 60 to 80 cubic feet in ordinary dwellings. If indirect radiation is used, these figures would be from 40 to 50 cubic feet. If hot water of high temperature is used, one square foot of radiation will take care of 50 to 70 cubic feet; and if low-temperature hot water is used, these figures should be from 30 to 50.