436 EVENING DISCOURSES 



differently from the eye in the matter of colour necessitates the use of filters 

 and other devices in photoelectric photometry. It is also possible to measure 

 the temperature of lamp filaments very accurately by means of a ' bridge ' 

 employing red-sensitive and blue-sensitive cells. (A demonstration of the 

 use of such an apparatus to detect slight changes in light-colour was then given.) 



Since legislation is now making it illegal to allow the emission of black 

 smoke from factory chimneys, it is fortunate that we have in the photo-cell 

 a means of continuously indicating (or recording) the density of the smoke in 

 furnace flues. A beam of light passes across the flue and falls on a photo- 

 cell connected in a valve circuit with an indicator placed in the furnace room. 

 The passage of dense smoke partially obscures the light, and the indicator 

 (perhaps reinforced by an alarm bell) gives warning that the stoking is not 

 being efficiently performed. (This principle was demonstrated with the aid 

 of a glass-fronted box into which smoke could be blown.) 



Thus far we have considered cases where the cell controls indicators, 

 counters, etc., but, with the assistance of gas-filled relays and contactors, 

 powerful machines may be actuated by light impulses. An electric door- 

 opener operated by the interception of a beam of light by a person approach- 

 ing the door is an example of this. When the person has both hands 

 occupied, such a device has very real value in saving time and labour. 

 {Demonstration.) 



Although a great many people are familiar with talking films, compara- 

 tively few are aware that without the photo-cell this method of sound 

 reproduction would be impossible. The passage of light and dark strips 

 recorded on the edge of the film, through a beam of light falling on a photo- 

 cell, causes minute variations in the current through the cell. These 

 fluctuations are amplified and passed into a loud-speaker, where they are 

 converted into sound. {Demonstrations of reproduction and of the special 

 ' recording lamp ' used for recording the sounds on the films were given at this 

 point.) 



By means of photo-cells used in much the same manner as that already 

 described it is possible to produce sound-waves of any desired wave-form, 

 or ' tone-quality.' {Demonstration.) Since it is largely the tone-quality 

 which determines the recognisable characteristics of musical instruments, 

 this principle may be of service in the construction of electrical musical 

 instruments. 



