Professor J.T. Way's Mercury Lamp

Although Humphry Davy demonstrated the possibility of producing light from a discharge in mercury vapour as early as 1821 by maintaining an arc between a wire and a pool of mercury, it is to the British inventor Professor J.T. Way that credit must go for the first demonstration of a practical lamp.  On September 3rd 1860 he completed an installation of modified carbon arc lamps on the Hungerford suspension bridge in London, in which the arc operated within a glass vessel containing air and a sizeable pool of mercury.  The actual point of light generation in the carbon arc is the incandescence of the electrode tips, the arc itself being substantially non-radiative.  But in J.T Way's lamp the heat from the discharge caused a small amount of mercury to evaporate, filling the glass chamber with its vapour, which was consequently ionised and imparted its blue-green colouration to the arc while also greatly increasing the total light output from this source.

Figure 7 - The Lamp of J.T. Way

He was duly granted a British Patent for this novel extension of carbon arc lighting technology.  One point worthy of mention is the method employed for starting the arc in his lamp.  With the ordinary carbon arc the discharge was struck by bringing one of the electrode tips into contact with the other, and then withdrawing it slightly, the arc initiating as soon as contact was broken.  This inconvenient method was surpassed in the mercury lamp, in which the upper electrode was fabricated with a small hole running along its length.  To strike the arc, part of the mercury was poured through this hollow electrode, falling into contact with the lower electrode and automatically initiating the discharge once the flow finished and contact was broken.  The electrodes in the lamp were stationary and as they burned away during use, complex electrical control gear would have been necessary to adjust the electrical supply as the arc gap lengthened.

The health and safety issues associated with such a lamp would be horrendous today, since mercury was freely vaporised and released into the open atmosphere.  Even their use outdoors would have been hazardous as a cloud of dense mercury vapour escaping from the lamps would have descended on passers by below.  No other known installations of Way's lamp are known, and in view of its dramatically enhanced performance this seems unusual.  It is probable that even in the mid 19th century there were concerns over the possible poisonous side-effects of the lamp he developed.  Certainly at this time it was becoming known that mercury was a prominent neurotoxin, attacking the brain and spinal cord.  Mercury compounds employed in the manufacture of felt, particularly favoured by hat-makers, were known for their undesirable side effects leading to premature dementia - hence the common phrase "mad as a hatter".