(i) Incandescent Vacuum lamps:
It consists of an evacuated glass bulb having of fine metallic wire within. Metal may be carbon, tantalum and tungsten. Used for low wattage.
(a) Carbon filament lamps: It is used for heating purposes
(b) Metal filament lamp
(c) Tungsten filament lamp
(ii) Gas filled incandescent lamps Carbon Filament Lamp:
Filling the bulbs with an inert gas such as argon or nitrogen. For wattage up to 1OOw, coiled coil lamps are used,
Specific resistance 0.001 – 0.0061 Ohm a – 0.0002to- 0 MP 3500°C
Specific resistance = 12.4 Ohm a = 0.0036 MP 2996°C Density 16.6
(c) Tungsten –
Specific resistance = 5.6 Ohm a = 0.0045 MP 3400°C Density 19.
Electric Discharge Lamp:
It has transparent enclosure and contains a gas or vapour at pressure. At the two ends of the lamp electrodes are provided for connecting the lamp to the main. Light is obtained from the excited atom of the gas.
Sodium Discharge Lamp:
It is in the form of U-glass tube, is enclosed in a double walled flask. Ii addition to sodium a small quantity of INERT gas (argon) is also inserted.
The colour is bright yellow and is recommended only for street lighting.
Mercury Fluorescent Lamps or Tubes:
Due to low pressure the lamp is in the form of long tube, coating of inside the tube is done with phosphor. For commercial use the phosphor usually contain a heavy impurity called activator.
The starter, filaments of the tube and the choke all form one series circuit. The series choke acts as a ballast when the lamp is running and it also provides a voltage impulse for starting.
Fluorescent bulbs in globe luminaries can be installed at the edges of exhaust hoods at regular intervals in kitchen.
Ii is used for illuminating sign boards; it emits orange pink coloured light. It consists of two electrodes which are made of pure iron. The electrodes are spaced few mm apart in spiral coiled lamp.
The tubes can be made in any shape. Different gases are filled in the tubes to get different colours such as argon for red colour, Neon with mixture of mercury for green colour and Helium for yellow colour. It works on high voltage, in the range of 2000 to 6000 volts.
High Pressure Mercury Vapour lamps:
(I) M. A. Type:
It consists of a glass tube of borosilicate which is quite hard. At the two ends in the tube are provided two electrodes of specially coated wire. Near the upper electrode is another auxiliary starting electrode which is connected to the bottom electrode through a high resistance.
The tube is sealed with an inside pressure of I’A atmosphere. The tube is further enveloped by another tube. The lamp has a screwed cap and is connected to the main supply through a choke. A condenser is connected across the main.
The inner tube in addition to mercury also contains a small quantity of argon gas.
(II) Mat Type:
The outer tube consists of tungsten filament in series with the discharge tube so that it acts as a blast. It can be used for ac as well as dc mains.
The colour given out by the MAT type lamp is of poor appearance since f1 * $ short of red colour while light given by this lamp consists of a mixture of due to discharge lamp. The colour so obtained is more soothing.
(III) MB Type:
This lamp operates at an extra high pressure of 5 to 10 atmosphere discharge tube in this type of lamp is of quartz. About 5 cm long has electrodes-two main and one auxiliary. This tube is in a pearl glass bulb.
There is a high starting resistance in series with the auxiliary starting electrode. This lamp generally has 3 pin level cap so that it may not be put in an ordinary holder since it requires a chock and a condenser
The colour given out by the MAT type lamp is of poor appearance since it is short of red colour while light given by this lamp consists of a mixture of lights due to discharge lamp. The colour so obtained is more soothing.
The arc is produced by bringing two carbon electrodes together momentarily and then separating them,. This process can be carried by hand or by automatic mechanism. Ballast is required in series because of the negative volt ampere characteristics of the arc. A resistance ballast for DC arc where as an inductance is used with AC arc.
The source of light is the incandescent electrode.
Such lamps are used in search lights.
The Halogen Hob:
This runs on electricity, and comprises five individually controlled heat zones, each of which has four tungsten halogen lamps located under a smooth ceramic glass surface. The heat source glows red, when switched on, getting brighter as the temperature increases.
When the hob is switched on, 70 per cent of the heat is transmitted as infrared light directly into the base of the cooking pan; the rest is from conducted heat via the ceramic glass. Ordinary pots and pans may be used on the halogen hob.
The halogen range includes a convection oven, and the halogen hob unit is also mounted on a stand.