Don't look for transformers much before the year 1900. The high-voltage transformer at the left above at Kenyon College was offered in the 1900 Max Kohl catalogue at 110 marks (about $25.00). The secondary coil is insulated with oil. With some additional parts, this is the basis for a Tesla coil. The high voltage transformer at the right above, from Transylvania University, has a spark gap attached to its secondary coil.

   The primary coil at the left below, from Vassar College, is all that remains from a smaller Kohl coil. It came with two removable secondary coils, which did not have oil insulation.

   The transformer of modern appearance is listed at $7.20 in the 1912 catalogue of the C. H. Stoelting Company of Chicago. The turns ratio is about 4:1 and it can be used as either a step-up or a step-down transformer. It is also from Vassar.

This set of primary and secondary coils, in the collection of St. Mary's College in Notre Dame, Indiana, is somewhat of a mystery. From its appearance, it was made in the first quarter of the 20th century as part of the oscillating circuit in a radio transmitter. The 1916 Knott catalogue shows a very similar device, but with only one coil.

The apparatus at the left is listed in the 1941 Cenco catalogue as "High Frequency Apparatus, Tesla Type", and sold for $80.00.    In use, an oil-filled capacitor is connected across the primary coil. This was run with a 10,000 V to 25,000 V transformer.    A similar apparatus in the 1916 Knott apparatus catalogue had a built-in high voltage transformer, and cost $160. The text notes that the system "will produce X-Rays of surprising power and penetration." This application is blatantly illegal today.

This is a uniquely twentieth century form of transformer: the coupling and tuning coils for a crystal set. The number of turns in the primary and secondary are selected by the rotary switches, and the mutual inductance between the two coils is adjusted by sliding the inner coil in and out. A complete crystal set using a similar tuning coil can be see on another page.    The apparatus is in the Greenslade Collection.