ELEMENTS OF ELECTRICAL ENGINEERING. 



139. Arc lamp globes, shades, and diffusers. The old style 

 open arc lamp is always provided with a globe to shield the arc 

 from drafts of air which are likely to blow the arc out, and the 

 hot enclosing bulb of an enclosed arc lamp is usually shielded 

 from gusts of cold air by an outer globe when it is used out of 

 doors. Thus Fig. 181 shows an enclosed arc lamp with a clear 

 glass globe for protecting the enclosing bulb. 



The intense brilliancy of the electric arc produces a glare that 

 is very unpleasant, even when an arc lamp is suspended at a con- 

 siderable height overhead out of doors. Therefore the bulb of 

 an enclosed arc lamp is nearly always made of opal glass, and 

 when an enclosed arc lamp is used for inside lighting the enclos- 

 ing bulb is generally surrounded by an outer opal globe or shade 

 for still further reducing the glare. Thus Fig. 187 shows an 

 enclosed arc lamp arranged for inside lighting with a shade 

 underneath the enclosing bulb and a large corrugated diffuser 

 above. 



Lamp globes are used not only to protect the arc and to 

 eliminate glare, but also to throw the light in a desired direction.* 



140. Series grouping of arc lamps across constant-voltage mains. 

 Where several arc lamps are to be operated continuously, that 

 is, where it is not desired to turn individual lamps off and on, the 

 lamps may be arranged in series groups if the supply voltage is 

 220 volts or more. Thus it is common practice to operate en- 

 closed arc lamps in series groups of two, across 22O-volt mains ; 

 and in series groups of five, across 5 50- volt mains. In such cases 

 it is necessary, because of the constant-voltage supply, to insert 

 ballast resistance in circuit to steady the lamps as explained in 

 Art. 137. Failure of one. lamp to operate, however, breaks the 

 circuit and puts the entire group of lamps out of service, and if 

 such a result is to be avoided each lamp must be provided with an 

 automatic cut-out arranged to establish a by-pass through an 



* A n exhaustive series of experimental studies of globes and reflectors is given in a 

 paper entitled "A Photometric Comparison of Illuminating Globes," by R. B. Wil- 

 liamson and J. H. Klinck, Journal of the Franklin Institute, Vol. 149, p. 66, 1900. 



