**L**eo Gratz was the first to investigate the dispersal of electrical waves. Thus the telephone, radio, and television are based on his discoveries as well as discoveries of Hertz.

Leo Gratz was a German Physicist born at Breslau, Germany on September 26, 1856. He was the son of the famous Jewish historian, Heinrich Gratz. Leo studied mathematics and physics at Breslau, Berlin and Strassburg.

In 1881 he became assistant to A. Kundt at Strassburg and in 1883 he went to the University of Munchen, where he became a professor in 1908 and occupied the Second Chair for physics parallel to Roentgen. His scientific work was first concerned with the fields of heat conduction, radiation, friction and elasticity. A non-dimensional parameter reflecting properties of a heat flow is named after him, Gratz number.

Portrait of Leo Graetz painted by Franz von Stuck, Oil on canvas, 1906

Gratz number (Gz) is proportional to { (thermal capacity) / (convective heat transfer) } and is used in heat transfer in general and convection in laminar flow calculations in particular. It is equivalent to {(L/d) / (Re.Pr)} or {(L/d) / Pe}. It is normally defined in one of the following forms:

Where:

alpha = Thermal diffusivity

Cp = Heat capacity

d = Diameter

G = Mass velocity

k = Thermal Conductivity

L = Length

m = Mass flowrate

rho = Density

V = Velocity

After 1890, his work forcused upon problems of electromagnetic waves and cathode rays. Gratz was a prolific technical writer as evidenced by his twenty-three editions of book “Electricity and Its Applications” and a five volume work “Handbook of Electricity and Magnetism".

These works contributed to the wide dissemination of knowledge in electricity which, at their time of printing, was still in its infancy. Leo Gratz was the first to investigate the dispersal of electrical waves. Thus the telephone, radio, and television are based on his discoveries as well as discoveries of Hertz.

He died in Munchen on November 12, 1941, at age 85.