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reflected back through or nearly through the source of light, and emerge from the lower part of the globe on the opposite side.

About 45 degrees from the top, the prisms deflect the up-going rays, so that they emerge horizontally and somewhat below the horizontal. From there down to the level of the arc the prisms all refract and reflect the light down toward the floor or street. Below the horizontal the function of the prismatic ribbings is to distribute the light so that the objects below the arc are uni

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formly illuminated, weakening the intensely bright zone due to the maximum candle-power about 40 degrees below the horizontal, and lightening the darker circle that tends to exist immediately below the lamp.

To secure still more perfect diffusion the globe is ribbed vertically inside, as shown by the horizontal section in Fig. 274. The effect of the diffusion is to make the outer edges of the globe, viewed from the side, appear as luminous as the center.

Holophanes have the disadvantage of requiring a stationary source of light to work to best advantage. This calls for a focusing open arc, or a fairly stationary inclosed one, two inches variation in a 12-inch globe not being excessive. They have the disadvantages of being somewhat expensive, heavy, and harder to clean, but are a marked improvement over the old style in diffusion and economy of light. To show the care exercised in the design of the prisms, the manufacturers of holophanes in this country state: "The large holophanes have as many as 400 calculated faces, each designed for a special duty. The profile of each one of these prisms is calculated by the laws of optics, and drawn on a very much enlarged scale to secure accuracy. The drawing is then reduced and transferred, by a photographic process, to a

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steel plate, and the profiles cut out with the accuracy of engravers' work. A tempered steel tool is then made corresponding to this template, accurate to the one thousandth part of an inch; and this tool is used in cutting the grooves in the mold in which the glass is pressed, after which it is annealed."

The cuts 275 and 276 show the effect of clear and opal globes on the distribution of light.

Lamp Mechanisms and Constructions. — The functions of an arc-lamp mechanism may be described as follows: —

1. To separate the carbons after having brought them into contact, if they were not together previously.

2. Maintain the distance between the carbons such that the energy at the arc is constant.

3. Feed one or both carbons together as they are consumed.

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Fig. 276. Illumination on Street Surface from Opal Globe.

4. Leave the lamp in such condition when the current is

turned off that it will resume operation when the current is renewed.

To these must be added another duty, dependent on the class of circuit on which the lamp operates, usually effected by the "cut out " in one of the following ways:

5. On constant current circuits, to maintain the continuity of the circuit through another path, if that through the carbons is broken by failure to feed or by reason of their having been used up or broken.

On con»tant potential circuits to open the circuit under the same conditions if the lamps run in parallel, or to substitute an equivalent resistance if two or more lamps run in series.

Regulation may be by hand or automatic. Hand regulation is used where the operator is always present, as, for instance, with projection lanterns, searchlights, etc. For arcs employed for general illumination automatic regulation is invariably used.

The general principle employed is the balance effected between the pull of a spring, gravity, or both against the pull of one or more solenoids or magnets. The variations in mechanical details are endless. The balance is preserved when the arc is in its normal condition. The mechanism is so arranged that too great length of arc will weaken the solenoidal pull, and too short length increase it or vice versa. In lamps intended for series circuits, these functions are performed by two types of mechanisms, known as shunt and differential.

In shunt lamps a circuit is led to the solenoid from opposite sides of the arc, so that the normal voltage across the coil, whose resistance may be 400 or 500 ohms, is 47 volts for an open arc.

When the current is off, the carbons are held apart by the retractile force of a spring, gravity, etc. On turning on the current, a high voltage exists across the gap between the electrodes, and the solenoid overcomes the retractile force, feeding the carbons together. When the carbons touch, the voltage instantly drops, and the retractile force, overcoming the weakened solenoid, pulls the carbons apart, and springs the arc. When the voltage rises too high, the shunt coil again feeds the carbuns enough to restore a balance. Two points are worthy of special attention in connection with shunt lamps. The first is that the carbons are apart at the start, introducing a very high resistance (450 ohms for each lamp in the series), unless there is an auxiliary cut-out circuit. This exceedingly high resistance introduces a difficulty in starting the average arc dynamo, and gives rise to potentials exceeding the line voltage, and possibly dangerous. The other feature is that the mechanism is entirely independent of current strength, and will maintain a given voltage across the arc whatever the current. Therefore such lamps will operate on circuits of various current values without any additional adjustment. They have also the property of varying the energy, and therefore the light at the arc no more nor less than the percentage that the current varies from normal. On the other hand, when the current of a line abnormally increases, decreasing the resistance of the arcs, and tending to grow still larger, these lamps do not assist the dynamo to regain its equilibrium. This gives rise to a tendency to unstable equilibrium of the current in the line manifested by surging of the lamps or jumping arcs.

The differential lamp when at rest has its carbons in contact. They are .separated by the pull of a series coil opposing gravity or a spring which the shunt coil assists. In this, as in the shunt lamp, the pull of the shunt coil tends to feed the carbons together. Obviously this lamp has a low resistance before the current is turned on, which is an advantage. The current passing through the carbons and series coil energizes the latter to pull the carbons apart, against the action of gravity, because the shunt coil is inert when the carbons are in contact with no voltage across them. This insures a rapid and positive opening of the arc. As soon as the arc is sprung, the shunt coil begins to act, but does not effect a balance until the series coil has pulled the arc long enough for the normal voltage. When the voltage rises, the shunt coil feeds the carbons exactly as in the shunt lamp. These lamps will work properly only within small limits on either side of the current to which they are adjusted. An increase in current will result in a stronger pull of the series coil, which will draw the carbons apart, until the increased voltage, acting through the shunt coil, again effects a balance; less current will weaken the series, and allow the carbons to approach. Such lamps therefore increase the apparent resistance of the arc as the current rises, and correspondingly assist the dynamo in maintaining the current value constant. For a given variation in current, however, they show a greater variation in light, since they increase the arc voltage with the current, so that the watts rise faster than the current strength. One point in favor of the differential lamp is that the striking of

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