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with a loud click from one position to another like the bottom of an oil-can when it
is pressed.
3.2 Noises Arising from Mechanical Effects
Mechanical noises may also arise from moving parts, such as the feed mechanism of an arc lamp. Rotation of the positive carbon and steady feeding of both positive and negative are essential conditions for the quiet and stable operation of the larger high intensity arcs and these movements are obtained from a small electric motor which drives and rotates the carbons through suitable reduction gearing. Quietness of operation can be secured by the use of well cut, well fitted gearing, and fibre gears have been recommended. It is also desirable to keep the rotational speed of Ithe motor as low as is practical, and to use adequate sound insulation in the mounting of the motor and gearing on the frame of the arc lamp. In the absence of such sound insulation the metal components of the arc lamp act as sounding boards and amplify considerably the noise arising from the motor and gearing. This point is illustrated in Fig. 14, which shows photographs of traces obtained by assessing the noise with the microphone-amplifier cathode ray oscillograph equipment. The first trace shows the noise emanating from a motor and gearbox solidly bolted to the frame of an arc lamp ; in the second trace there is a record of the same motor and gearbox running under precisely the same conditions except that in this case the motor and gearbox are acoustically insulated from the lamp frame.
A further reduction of the amount of noise escaping from the lamp house can be obtained by the use of pads or blankets of sound absorbing material placed in appropriate positions so that they will absorb sound incident upon them, but will not in
Fig. 14. Noise from motor and gearbox of arc lamp feed, (a) Motor solidly bolted to arc lamp frame, (b) Motor acoustically insulated from arc lamp frame
[terfere unduly with other functions of the lamp house such as the ventilation and cooling2.
Illuminators using incandescent lamps have no parts which move when the lamp is switched on and so are free from this source of noise.
3.3 Noises of Electrical Origin
The fundamental phenomena which give rise to noise of electrical origin are two in number. The first of these is the flow of electric currents, which is always accompanied by mechanical forces. For example, any two parallel cables in which current is flowing in the same direction will attract each other ; if the current flows in opposite directions — and this is the usual case — then repulsion will occur, and the magnitude of the forces will depend upon the magnitude of the current, the distance between the conductors and their length. Thus in the case of two parallel cables the force between them is given by :
F = 2Hhl r where F = Force between the cables.
h>h = The currents carried by the two conductors / = Length of the conductors r = Distance between the conductors The second is that no practical D.C. generator will generate a perfectly steady voltage and so there will not be a perfectly steady flow of current unless steps are