Acoustic microscopy acoustic microscope (CSAM), and C-mode scanning acoustic microscope (C-SAM). More recently acoustic microscopes based around picosecond ultrasonics systems have demonstrated acoustic imaging in cells using sub-optical wavelengths working with ultrasonic frequencies into the multi-GHz. Since the vast majority of acoustic microscopes in use today are C-SAM type instruments, this discussion will be limited to these instruments. Ultrasound is broadly defined as any sound having a frequency above 20 kHz, which is approximately the highest frequency that can be detected by the human ear. However, the acoustic microscopes emit ultrasound ranging from 5 MHz to beyond 400 MHz so that micrometre
History of the Tesla coil New York for researching them, initially repeating Hertz's experiments. He first developed alternators as sources of high frequency current, but by 1890 found they were limited to frequencies of about 20 kHz. In search of higher frequencies he turned to spark-excited resonant circuits. Tesla's innovation was in applying resonance to transformers. Transformers functioned differently at high frequencies than at the low frequencies used in power systems; the iron core in low frequency transformers caused energy losses due to eddy currents and hysteresis. Tesla Tesla invented the Tesla coil during efforts to develop a "wireless" lighting system, with gas discharge light