246. Globes

Globes. The primary object of enclosing a gas burner within a globe is to protect the flame from interference by drafts of air. A globe, however, acts as a chimney and causes a strong upward current of air to flow through it. If the dimensions of the globe are not suited to the size of the flame, the air-current will cause the flame to flicker badly, and the globe then becomes a detriment instead of an advantage.

The opening at the top or bottom of a globe should never be less than 4 inches in diameter for an ordinary 5-foot burner, and a larger size is still better.

Globes are often required to serve the purposes of shades, to modify and soften the light. For this purpose the outer surface of the glass is etched, or ground, or colored glass is employed. These globes obstruct the light, the loss being about as follows:

Globes of clear glass obstruct the light somewhat; but, if a globe is properly proportioned, the intensity of the light will be increased by the draft which it creates, and the increase of light will counterbalance the loss by obstruction.

247. Shades are commonly used to obstruct the light from passing upwards, and to reflect a considerable part of it downwards. It is, therefore, desirable that the under side of the shade should have a good reflecting surface. For table and desk lights, the shades should be made of opaque material, and should act as reflectors only.

The color of globes and shades is a matter of some importance. If translucent shades are used, they should be either white or opal. Red, green, and blue shades should not be used, because of the bad effect of the colored light upon the eyes, red light, especially, being very tiresome to sensitive persons.

The central opening in the top of the ordinary shades permits a considerable portion of the light to pass upwards to the ceiling. When it is desired to prevent this, top reflectors may be used to intercept the light and throw it downwards, as shown at a in Fig. 93.

248. Shields.not be permitted at a distance less than eighteen inches below the ceiling.

The distance between an ordinary gas burner and the ceiling should be not less than three feet. If a less distance is unavoidable, the ceiling should be protected by a metal shield to prevent it from being scorched or burned. The shield should be separated from the ceiling by a clear space of at least two inches, to permit air to circulate between them. Even when a shield is provided, a gas flame should 4-11

Fig. 93.

If a gas flame is liable to come into accidental contact with inflammable materials, such as curtains or drapery blown by the wind, or hay and straw in stables, etc., it should be provided with a glass globe, and should also be enclosed within a stout wire cage at least ten inches in diameter. The only safe way to determine the proper size of the wire guard is to test it by holding pieces of cloth or paper against it. If the material can be set on fire, the guard should be made larger.

249. The discoloration of ceilings may be mitigated, although not wholly cured, by using a deflector, as at a in Fig. 93, or by hanging an ordinary smoke-bell over the flame. By spreading out the current, its velocity is checked, the amount of dust which strikes the ceiling within a given area is thus reduced and the discoloration is lessened.

The only effectual method of preventing the discoloration of walls and ceilings in this manner is to intercept the current of hot air arising from each burner, and to conduct it to a chimney or ventilating flue, by means of a hood suspended over each flame, or set of flames, and suitable pipe connections. This plan is valuable for another reason. Not only are the products of combustion removed from the room, but a considerable amount of air is carried along also, thus aiding ventilation. The hot gas which is discharged into the ventilating flue raises the temperature therein, and thus increases the draft and improves the operation of the flue.