The Spark Transmitter
Photo of a small spark-gap transmitter, an early type of radio transmitter used during the wireless telegraphy era before 1920. This example is a small demonstration unit from a 1909 book on radio. The source text says that in a practical transmitter the small inductor (coil) shown would be replaced by a larger coil. From Walter Wentworth Massie and Charles Reginald Underhill (1909) Wireless Telegraphy and Telephony, Popularly Explained, D. Van Nostrand, New York, p.19, fig.7 on Google Books
The origins of wireless telegraphy and radio hinged around the use of spark transmitters. These transmitters were used in the VLF and LF frequency bands. One characteristic of the spark tramitter was its ability to produce a lot of unwanted interference (in todays crowded bands where spectrum efficiency is important), With the advent of the vacuum tube, spark transmitters have been relegated to history.
An engineering tuturorial circa 1936