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Photoelectric Tubes and Unit Light Source >** Photoelectric rub# he* 4 Boom on corhodoeoutei A A unit light source I* on equal distance from oil the pomt* crj o eothode ond an onode 0 electron flow .to anode ** a sphere so that the Intensity of the light can be meowmlj calcium and their oxides--end metals --auch as tantalum, titanium and thorium--are used (or cathodes ' of special-purpose photoelectric tubes. Sodium might be used as the light, sensitive element in s tube sensitive to the ultraviolet end ol the spectrum. Dut an ultraviolet-sensitive photoelectric tube, not sensitive to vitiblo light, would have a tantalum*, titanium- or thoriumcoated cathode. Light Flat. When we discussed hot* cathode tubes we learned that with other conditions constant, electron Row through the tubes varied as the current flow through the cathodes. In other words, lor a given anode voltage, elec* iron Row through the tube Increased or decreased as the cathode's temperature was Increased or decreased between limits. Similarly, In a photoelectric tube lor given anode voltage, electron Row through these tubes is determined largely by the amount oi light reach ing the cathode. Light is a medium of energy transmission just as on electric current in s circuit is. But wc do not speak of light flow os wo do of an elec* trie current: we coll it light flux or light quantity. Since light is one ol the most im portant factors when dealing wfth pho toelectric tubes, we should have some Idea how it is measured. The three most commonly used units are candlepower, loot-candle end lumen. A candlepower la the unit of luminous in* tensity or unit source of light. It is approximately equal to the light from a %-in. sperm candle, burning at a rate of 120 grams, or 0.274 os per hour. Light is given ofl from the source in all directions. To have the light source an equal distance from all points oq s surface so its intensity can be meas* ured at a given distance, the source is placed in a sphere as in Fig. 4. The standard sphere has a l*ft radius or 2-ft diameter, as indicated. Then, all points on tho sphere's inner surface are 1 ft away from the light source. Under this condition (he lumlnous*flux density on each square fool of surface is ealled a foot-candle. In other words, e one-candlepower light source will emit one foot-candle to each square loot of surface one foot away. A tuminoua-flux density of one foot-candle is a lumen per square foot. Since a sphere 2 ft in diameter has 12.57 sq ft area, a one-andlepower light source emits o total light flux of 12.57 lumens. Assume wo put the one-candlepower fight into a 4*ft sphere. We would still have the same quantity of light flux, 12^57 lumens, from the light source but it would bo dispersed over four times the area. Now the luminous*flux density will be only V4 fool-candle. From i we see that while the total tight ftm] from a source remains constant, flu density per square foot, foot-candles varies inversely as the square of i` distance from the sourco--4hal is, if i double the distance the light internUjJ decreases to one quarter per unit in' tloctric-Lomp Application. How del all this apply to an electric lamp? A 7 wall tungsten gaafilled lamp wii^ frosted bulb is rated 1100 lumens, mating sucb a light source at the ew of a sphere 2 ft in diameter, the an of luminous flux on each iquare-fw area, would bo 1100 - 12J>7, or ah 88. Foot-candles equal lumens square foot; therefore, we have a itu nous-flux density per square foot of 1 foot-candles. At 10 ft from the lamp, we have l same total luminous flux, as at onedofj distance, but it is spread 10x10 = 100 limes the sree. ConJfi quently the luminous-flux deadj equals B8 *+* 100 = 0.88 foot-coodla ** 0.88 lumen per square foot We ther fore do not havo to get very fer stn from an ordinary tight source befoj the intensity drops to a low value. The next article wilt show how V' from an elecirio lamp Is concentrtK into a beam, how this beam is maalR lated to cause photoelectric tubes | perform many services, 109 (494) POWER Auqui' Extra Valve Improves Maintenance Procedure Clean Out Nipples Without Read Tank-Water Level' Shutting Down tho Main From Pressure Cage Hydraulic tuhbINKS served by single penstocks usually have butterfly valves si the spiratcasing inlet. Whenever the turbine is down lor repairs, this volte shuts off the water without un watering the penstock. Standard prac tice -provides a bypass with a single shutoff valve around the butterfly valve. Before starting-up the turbine this by pass permits Ailing the spiral casing with water, thereby equalising the pressure oo both sides of the butterfly wicket for greater ease in opening. For an inatollation with 64-in. butter fly valves under 413-fl head with very frag penstocks, it was realised that con siderable advantage could bo gained by providing duplicate guard valve in .by pass. Extra goto valve, upstream of con ventional goto voire, is normally open. With both guard valve snd butterfly vslrs closed, however, tbe regular by* pass gate vU tan be removed readily lor inspection sad repair. Without the guard valve, the regular valve could be removed only after unwalering the. entire penstock, often leading to neg lect of minor repairs. Enwaao Urhunc Milwaukee, Wix. A NUMuen of our steam-flow meters were indieating incorrectly because the meter nipples in the steam main had scaled up. We designed the tool shown above, which has permitted reaming out these nipples without taking the steam main out of service. The unit is built around a 4-in. tee. At one end of the straight run attach a abort nipple with s cap drilled to accommodote the dutch-drill rod. At the back end of tho cap place a soft ring of packing with a steel washer to prevent leakage. At the other end of the tee, attach the gate valve and nipples as shown. To the tee branch fix a nipple and blowoff valve. Pull tho drill back behind the gate valve and close both valves. Uncouple meter line from shutoff valve on meter nipple and attach unit. Open both gate snd meter valves and operate .drill rod with tap wrench or suitable too). As reaming progresses the blowoff valve can be cracked to remove scale pani cles. All items are standard except the dutch drill, which can be made easily from any piece of standard drill rod. E Poomavka Palisade* Park, N. J. The tiDtars Invite ContrSuttanis 10 b on one of ois'I itrlieeki"a yo"u h"av"e used. a pobii.iTj\owu *20 for <* hd here on problems In do 'OWt, . 1 Avqvit 1946 sign, operation, maintenance. Your con tribution need be io rough 'form only-- Joit set down all facts wfih sketches in pencil. We will do any editing needed. Hydrostatics, when properly applied, can give a convenient means of indicat ing the water level of an elevated tank for water storage. At one plant a standard elevated tank was erected so that its bottom stood 75 ft obrive the ground. Its top was 100 ft shove the ground. On tliU installation an ordinary wa* ter-pressure gage was piped Into the existing system and placed at the base of the tank. Then a curve chart was made up so (he operator could tell lank level by ptessute readings. Here Is how it works. When the tank was full, water level reached the top of the tank and hence stood 100 fl above the ground. Knowing that a pressure of 1.0 psi U re quired to support a column of water 2.31 ft high, all that need be dono is to calculate the gage pressure for a 100-ft column, or 100 ft divided by 2.31 ft gives 43.31 pJ--say 43. At the empty position, the gago pressure fig ures 75 divided by 2.31, sod is 32.4 pal or 32. The curve chart that is shown above translates the pressure reading to the. tank level You con go still one stop farther. With the total tank capacity known (10,000 gal) capacity per ft of a partially filled tank ean be figured out. In this case 10,000 gal divided by 25 ft gives 400 go) per foot of tank. J D CottSTAwer. CUffride Park, TV. /. (Continued on page 110) j;W *f0 '(495) 109