A Spectrometer for Electromagnetic Radiation 
193 
placed immediately behind it. The radiation directly received at 
R interferes with that reflected from M. By shifting M back about 
2 cm. at a time the phase relation of the two waves is changed and 
a system of nodes and loops obtained. In Hertz’s original experi- 
ment a mirror 13 feet high was used; with our apparatus we com- 
monly use one about one foot square. Fig. 3 shows the results of 
such an experiment with a mirror only 4 inches square. Distances 
of the plane mirror behind the receiver are plotted as abcissae and 
the corresponding galvanometer deflections as ordinates. Two 
maxima and minima are well shown. With a mirror i foot square 
3 maxima and 4 minima appear. (See Phys. Rev.,yo\. 23, p. 244.) 
The use of a thermojunction receiver (whose indications are 
proportional to the energy received) shows the rate of damping- 
out of the stationary waves, as well as the position of the nodes. 
