Mills Novelty Company
Early Relay and Resistor Piano Expression

(Photograph courtesy of Art Reblitz)

Mills Novelty Company early piano relay/resister expression system electrical schematic. In the above schematic, the music roll feeder assembly is represented by the circular drums at the drawing's right side, with the contact roller shown as grounded. Thus, when a music roll perforation moves over the grounded contact roller a tiny wire brush makes an electrical contact, thus actuating some particular function.

The "upper bus" piano magnet coil shown (near the center top of the schematic) is representative of ALL the piano magnets included in that electrical bus. The same analogy is true for the so-called "lower bus," the two buses then essentially dividing in half the electrical load for operating the piano note scale. When a brush connected to a piano magnet, in either the "upper" or "lower" bus, makes contact the corresponding piano note plays at a moderate or "medium" intensity.

The "M" at the upper left of the drawing is the positive voltage feed, whereas the feeder contact roller is the ground return for any electrical circuit. Because of the dropping resister, represented by R1, R2, R3, and R4, the voltage to piano magnets is reduced, mainly by R1 for the "upper" bus, and by R4 for the "lower" bus. This voltage drop causes the piano notes to play at a moderate intensity. This is the default state of the piano expression system.

When  the "loud" relay is energized (by music roll perforation #120) the dropping resister is effectively removed from the circuit, and so any piano note energized operates at full voltage, and therefore plays at full or loud intensity. For soft notes, music roll perforation #122 grounds the center tap of the resistor network. This causes increased current flow through resistor segments R1 and R4, which results in a further and significant voltage drop, thereby causing any piano magnets energized to play at low or soft volume. The amount of additional voltage drop for the soft setting is determined by the value of resistors R2 and R3. The lower the resistance of R2 and R3 the greater the current flow and subsequent voltage drop across resistors R1 and R4, and conversely the greater the resistance of R2 and R3 the less the voltage drop.