Like other machines for medical electricity, Davis's devices consisted of a step-up transformer (a double helix) with some means of driving the primary with non-direct current. The helix at the right, below had a rasp for interrupting the direct current applied to the primary, but could also be used with the electromechanical interrupter shown in the cut at the left, below. This is Charles Grafton Page's revolving armature, here used because its commutator interrupted the current driving it every half turn. This is shown in the 1842 edition of the Manual of Magnetism, pg 9 of the appendix of medical applications of electricity. The instrument is at the Collection of Historical Scientific Instruments at Harvard University.
 

This double helix with vibrating electrotome is not shown in this form in the Davis catalogues, but a very similar version with a rasp on top is shown for $10.00 to $12.00 with battery and shocking handles. It is in the collection at Washington and Lee University.