15) How many lamps of 3 volts rating are required to be connected in series to a supply of 200 V for satisfactory operation ?

16) The most popular type of fluorescent lamp for new installation is the :

17) The type of fluorescent lamp that can be used in flashing circuits is the :

18) In a H.P.M.V. lamp, the mercury vapour discharges at :

19) Most recent development in fluorescent lighting is the :