Since the flowing medium is ordinary tap water which is an electrolyte, gas bubbles can
be generated by the electrolysis of water. The electrolysis takes place when two
electrodes are immersed in water and a voltage applied, between them. Oxygen is
evolved at the anode and hydrogen at the cathode. The hydrogen bubbles so produced
are much smaller in size than the oxygen bubbles and the volume of hydrogen formed
is twice that of oxygen. Hence the hydrogen bubbles are used as tracer particles for
visualisation. If the cathode is a fine wire small hydrogen bubbles are produced from the
wire. Therefore the cathode is selected as the bubble producing wire and the anode
may be any conducting material or terminal immersed elsewhere in water. Normally the
cathode is a very thin wire of diameter 0.05 mm or less. Very thin wires produce very
small hydrogen bubbles in size and the buoyancy forces become negligible compared
to hydrodynamic drag forces causing little disturbance of actual flow conditions.