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ELECTRIC LIGHTING, PROGRESS OF. 



3. Illuminating Power. The illuminating 

 power of the lamp was measured on a Bunsen 



Ehotometer. 'At one end of the bar was the 

 imp itself; at the other end two standard 

 candles, placed nearly in line. The plane of 

 the carbon filament was placed at 45 to the 

 length of the bar, and each lamp was measured 

 at 16 and 32 candles. 



III. APPARATUS EMPLOYED. 1. Condenser. 

 The condenser used in these measurements 

 had a capacity of 1 microfarad, divided into 

 sections of 0-4, 0*3, 0'2, and 0-1. The dielectric 

 was paraffined mica, and the brass-work was 

 supported on ebonite pillars. 



2. Galvanometer. The galvanometer was a 

 Thomson double-coil astatic instrument, in- 

 closed in a square case with glass sides. Meas- 

 ured resistance, 6,550 ohms. Used with lamp- 

 stand and scale, in the ordinary way. 



3. Standard Cell. An ordinary Daniell cell, 

 the copper plate being immersed in a saturated 

 solution of pure copper sulphate, contained in 

 the porous cell, and the zinc plate amalga- 

 mated, in a saturated solution of pure zinc sul- 

 phate, in the outer jar ; one of a battery of ten 

 cells forming a part of the Edison exhibit. 



4. Resistance Coils. A set of standard coils, 

 measuring from 1 ohm to 5,000 ohms. All 

 other resistances employed were standardized 

 by these. A set of coils used in the "Wheat- 

 stone's bridge. Compared carefully with set. 



5. Wheatstone^s Bridge. Four conducting 

 wires of large size arranged on the table in the 

 form of a rhomb. A test galvanometer was 

 inserted between the obtuse angles of the 

 rhomb, and a pair of shunt wires from the 

 main conductors were attached at the acute 

 angles. The first side of the rhomb contained 

 the lamp to be measured, standing in its place 

 on the photometer. The second side contained 

 a fixed resistance of 5 ohms. The third side 

 contained a variable resistance, and the fourth 

 side a fixed resistance of 950 ohms. 



6. Photometer. The photometer employed 

 was of the Bunsen form, having a double bar, 

 eighty inches long, graduated in inches and in 

 candles. The disk was of paraffined paper, 

 with a plain spot in the center. The disk-box 

 was movable on rollers, and contained inclined 

 mirrors to facilitate the adjustment. The can- 

 dles used were of spermaceti, made to burn 

 120 grains (7'776 grammes) per hour. The 

 entire apparatus was surrounded with heavy 

 black cloth. 



7. Dynamo-electric Machine. An Edison 

 sixty-light machine was used to furnish the 

 current required. In this machine the field - 

 magnets, which are very long and heavy, stand 

 vertically. The field is maintained by a shunt 

 current, regulated by an adjustable resistance 

 in its circuit. The bobbin is wound on a cyl- 

 inder like that of Siemens, from which it dif- 

 fers, however, in its details. Its resistance 

 was only 0*03 ohm, and the current delivered, 

 at a speed of 900 revolutions, had an electro- 

 motive force of 110 volts. 



IV. RESISTANCE OF LAMPS COLD. The re- 

 sistance of the lamps cold was measured on a 

 Wheatstone's bridge of the ordinary form and 

 in the usual way. Twenty-four of each were 

 taken (except the Lane-Fox, of which only fif- 

 teen were furnished), and ten selected from 

 these for the tests. 



V. METHODS OF CALCULATION. 1. Illumi- 

 nating Power. The standard candle should 

 burn 7'776 grammes spermaceti per hour, or 

 0-1296 gramme per minute. The two candles 

 used should burn 0'2592 gramme per minute. 

 The corrected candle-power of the lamp, there- 

 fore, is obtained by the proportion : As 0'2592 

 is to the amount actually burned per minute, 

 so is the observed candle-power to the cor- 

 rected candle-power. 



2. Resistance (hot). From the theory of the 

 Wheatstone bridge, the resistance of either 

 side is equal to the product of the adjacent 

 sides divided by the opposite side. In the 

 bridge used for the measurement, the resist- 

 ances in the two adjacent sides were 950 and 

 5 ohms. Hence, by dividing their product, 

 4,750, by the reading of the variable resistance 

 observed, the resistance of the lamp hot is ob- 

 tained. 



3. Electro-motive Force. In Laws's method 

 the electro-motive forces are proportional to 

 the multiplying power of the shunts employed. 

 Since with the Daniell cell no shunt was used, 

 the multiplying power of the shunt used with 

 the lamp - current represented directly the 

 electro-motive force through the lamp, in terms 

 of the standard cell. The multiplying power 

 of a shunt is the sum of the galvanometer re- 

 sistance and the shunt resistance divided by 

 the shunt resistance. In this case the resist- 

 ance of the galvanometer was 6,550 ohms. 

 Hence, if S represent the resistance of the 

 shunt, obtained by experiment 



6550 + S 



S 



will represent the electro-motive force. Since 

 the electro-motive force of a Daniell cell is 

 not 1 volt, as here assumed, but 1-079 volt, 

 strict accuracy would require the figures given 

 to be increased in that ratio. Moreover, the 

 small error arising from the inductive action 

 of the needle on the galvanometer coils has 

 been regarded as unimportant. 



4. Current. By the law of Ohm the cur- 

 rent strength is the quotient of electro-motive 

 force by resistance. Dividing the electro-mo- 

 tive force in volts by the resistance in ohms, 

 the current strength is obtained in amperes. 



5. Electrical Energy. The work done by 

 a current is proportional to the product of the 

 square of the current strength into the resist- 

 ance of the circuit. Or, since the electro-mo- 

 tive force is equal to the product of the cur- 

 rent strength by the resistance, the energy is 

 represented by the product of the electro-mo- 

 tive force in volts by the current strength in 

 amperes. This gives the energy in volt-amperes. 



