The Gramme Ring: A Continuous Winding for Steady Current

Print a ring 60 mm across with a 15 mm square section, on a hub that fits a steel shaft. Wind enamelled copper wire round the ring in eight equal coils of 40 turns each, all in the same direction and without cutting the wire between them: one continuous winding. At each junction between coils, pull out a loop and scrape its enamel. Print a commutator drum on the shaft with eight strips of copper sheet glued round it, insulated from each other, and solder each junction loop to its own segment in order.

Mount the shaft in bearings in a plywood frame with neodymium magnets on either side of the ring, one north-facing and one south-facing. Bend two strips of copper as brushes to press on opposite segments, at the positions where the output is greatest (find them by turning the brushes round while measuring). Spin the shaft with the cordless drill and read DC volts across the brushes. Switch the multimeter to AC volts: the reading is the ripple left on the DC. Now lift four adjacent junction loops off their segments, so the machine behaves as a coarser one, and read the AC ripple again — it rises. Compare with the two-segment Brush dynamo in this catalogue: same idea, fewer segments, more ripple.

**US 120,057, 'Magneto-Electric Machines', Zénobe Théophile Gramme and E. d'Ivernois, dated 17 October 1871.** Gramme's machines, built in Paris, supplied electroplating works and arc lighting in the 1870s; at the Vienna Exhibition of 1873 a Gramme machine was shown driving another as a motor. The ring was later replaced by the drum armature, which puts all its copper in the field (on a ring, the turns inside the ring do no work), but the continuous, many-segment commutated winding is Gramme's. **Honest limits.** The inner half of every turn on a ring is wasted copper. Commutators and brushes spark and wear. And a DC generator's voltage cannot be stepped up with a transformer, which is why alternating current won long-distance supply.