Induction Coil (ca. 1900) |
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At least until 1910 or so, the high voltages (tens of thousands of volts)
required for x-ray tube operation were usually provided by induction
coils. Induction coils operated off DC current, typically provided by a
battery of cells. Induction coils were eventually replaced by transformers
which operated off the AC current.
An induction coil actually consists of two separate coils. The inner "primary" coil consists of insulated wire wrapped around a central iron core. The outer "secondary" coil is wrapped around the primary. When current is applied to the primary coil, a magnetic field is created. The secondary coil is exposed to this magnetic field. The voltage induced in the secondary coil is greater than that in the primary. The ratio of the voltages is the same as the ratio of the number of turns in the two coils. For example, if the voltage provided to the primary coil is 10 volts, and the number of turns in the primary and secondary coils are 200 and 400,000 respectively (2000 to 1 ratio), the induced voltage in the secondary coil is 20,000 volts. The voltage is estimated by the length of the spark across the secondary spark gap. Coils were rated by the length of their spark (e.g., 8", 12", 20" etc.). |
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| A steady current in the primary does not induce a voltage in the secondary. This will only happen when there is a change in the magnetic flux created by the primary. To induce a current in the secondary, the current in the primary is rapidly turned on and off. This is accomplished by a device known as an interrupter. In the simplest designs, like the one shown here, the interrupter was an integral part of the coil. In the more sophisticated systems, the interrupter was a physically separate unit. The interrupter shown here works in the following manner. |  |
| When current is applied to the primary coil, the iron hammer (a) on the end of the contact arm is pulled to the magnetized iron contact (b) at the end of the primary's iron core. This breaks the interrupter's contacts (c) which stops the flow of current to the primary. Since the core is no longer magnetized, the contact arm then returns to its normal resting position. When this happens, the interrupter's contacts are closed again and current flows to the primary. The result is a rapid turning on and off of the current to the primary. It is the repetitive establishment and collapse of the primary's magnetic field that induces the high voltage in the secondary coil. | |
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