From the Rolls-Royce experimental archive: a quarter of a million communications from Rolls-Royce, 1906 to 1960's. Documents from the Sir Henry Royce Memorial Foundation (SHRMF).
Guide on two methods to test the candle-power of vehicle lamps to comply with lighting regulations.
| Identifier | ExFiles\Box 61a\1\ scan0032 | |
| Date | 2nd March 1915 | |
| The Motor 114 2nd March, 1915 HOW TO TEST THE POWER OF LAMPS. The Power of Four Candles—Do Your Side Lights Exceed This?—If They Do You May be Contravening the Lighting Regulations. IN the course of a case heard at Mortlake last week it was definitely decided that any lamp of greater power than that of the ordinary oil lamp carried by a taxicab would be considered a powerful light within the meaning of the regulations now in force. The light given by such a lamp is approximately the equivalent of two candles. The motorist, therefore, has now a definite standard to which he must endeavour to conform when motoring in the area subject to these special restrictions. The difficulty, of course, is to know exactly whether the lamps are of a greater or less power than that specified. In order to assist our readers in arriving at a decision on this point, we now describe two methods whereby the power of a lighting equipment may be ascertained. These are both so simple that they can be carried out at home. The candle power as standardized in this country is the light given off by a sperm candle of such a size that six go to make a pound. For our present purposes the ordinary paraffin wax candle of about the size mentioned would be sufficiently near to standard. Method No. 1. For the first test, all that is necessary besides the candle and the lamp on trial is a pencil or ruler—something which can be stood on end within a few inches of the wall. On the wall should be pinned a sheet of white paper as a screen. Exactly 10 ins. away from the candle should be placed. If the lamp be now stood by the side of the candle at an equal distance from the wall, two shadows of the pencil will be thrown on the screen of paper. The lamp, which will be probably the more powerful, will throw a correspondingly darker shadow. It is now necessary to draw the lamp farther away from the wall until the shadow caused by it is of equal intensity to that caused by the candle. The relative distances of candle from the wall and lamp from the wall are a measure of the lighting power of the lamp. It should be noted, however, that this ratio is not a direct one. If we call the distance from the lamp to the wall b, and the distance from the candle to the wall a, then the candle-power of the lamp is equal to b² divided by a². For example, we have placed the candle 10 ins. from the wall. If, when the shadows are equal, the lamp is 30 ins. away, the candle-power of the lamp is 30 x 30 / 10 x 10, that is to say—nine. For the lamp to be in conformity with the regulations, this quantity must be two, and its distance from the wall, when the above conditions are satisfied, should be 14⅛ ins. The lighting power of such a lamp is, of course, equal to 14⅛ x 14⅛ / 10 x 10, which equals very nearly two candle-power. The second method, which is only slightly less simple than that described above, is a modification of the one used by the R.A.C. a few years ago, when they made comparative tests of the lighting power of many headlights of different makes. The following little experiment illustrates the principle which underlies this test. Take a piece of white blotting paper and drop on to it a little grease from the paraffin candle. Scrape off the superfluous wax, and warm the paper so that the grease may thoroughly permeate it. The grease spot should be about the size of a half-penny. If this piece of paper be held up to the light, the grease spot will show as a bright circle in the centre of a dark sheet. The observer should now stand so that he has the light behind him, and looks on the side of the paper nearest the light; the appearance will now be the reverse of that previously noted. The grease spot will now appear as a dark circle in the centre of a white sheet. Such a piece of blotting paper should be secured to a frame or between two pieces of cardboard, as illustrated in our sketch. This should be placed on the table or floor, with a candle on one side, about 10 ins. away from it. The lamp to be tested must be placed on the other side of the screen, and so arranged that its flame is the same height from the table or floor as that of the candle. If we commence by standing on the same side of the screen as the lamp, and putting the lamp quite close to it, the effect of the light of the candle on the other side will be overbalanced. The appearance will be the same as if the candle were non-existent, and the grease spot will show as a dark circle in the centre of the white sheet. The lamp should now be gradually drawn farther away from the screen, and it will be noted that the dark spot will eventually merge into the background. A little farther still, and the effect of the candle light will predominate, and the grease spot will show light. Method No. 2. Diagram 1 labels: White Sheet Pencil or ruler 10" - (a) Ordinary candle Lamp on trial -b- Diagram 2 labels: White blotting paper Paper fastener Grease spot Cardboard Frame 10" - (a) -b- | ||