LARGE DOUBLE-CURRENT VOLTA-F ARABIC APPARATUS. 247 



3. The commutator E, represented iu fig. 55, but not shown in 

 fig. 52, is intended to transmit, rapidly and alternately, the current 

 either of the first or of the second coil to the conductors attached 

 to the knobs P and Q. It is unnecessary to describe it here ; and 

 it is enough to say that, by turning the needle F (fig. 55) to the 

 right or the left, it conducts one or the other current to the rheo 

 phores ; as, indeed, is shown upon a plate situated above the 

 needle. 



4. The graduator B (fig. 52) is a cylinder of copper, which 

 surrounds the coils, and which has a scale marked upon its upper 

 part. The knob B', fixed to its extremity, is a handle by which it 

 can be drawn out or pushed back. 



5. The moderator is a glass tube, F (fig. 52), terminating below 

 in a metallic basis to which is attached a knob, I, and above, in a 

 collar, K, from which proceeds a hook, which serves to connect the 

 moderator with one of the knobs, P, which receive the conductors 

 to the rheophores, and in which the poles of the coils terminate. 

 In the collar is a small opening, traversed by the stem of the 

 moderator J. The tube is filled with water. 



6. The trembler is composed of a piece of soft iron, A (fig. 52), 

 and of a platinized screw, S, against which the soft iron is pressed 

 by a small spring. 



The pedal Y (fig. 52), which replaces the toothed wheel, D (fig. 

 51), is to allow slow intermissions to be produced with the foot; 

 an arrangement which leaves the hands of the operator free, either 

 to hold the rheophores or to graduate the currents. 



8. The magnetic rheometer measures the intensity of the initial 

 current. It is a compass divided into four parts, of which each is 

 subdivided into ninety degrees. The rheometer no longer forms 

 part of the apparatus. 



B. — Large uncovered volta-faradic apparatus. 



I have represented in fig. 55 an instrument which has its coils 

 exposed. It differs from the common form (fig. 52) in the manner 

 following : — 1. It has no external covering, so that we may see the 

 arrangement of the coil A, and the movement of the cylinder B 

 over it. 2. It possesses two graduator tubes ; one, B, which acts 

 u]3on the second coil ; and another, C, which disables the primary 

 coil. 3. Its core of soft iron, D, is movable, and may be with- 

 drawn (fig. 56), so that we may study the influences of the tubes 

 B and C, independently of temporary magnetization. 4. Besides 

 the commutator of the coils, E, it possesses a commutator of the 

 poles, H, forming part of its structure, and by which the direction 

 of the currents may be rapidly changed without displacing the 



