30 Mr. Shelford Bidwell on a Modification 



In the first form two equal cylinders, one of wood and the 

 other of brass, are mounted on parallel axes, and so arranged 

 that a fine wire can be wound off the one and on to the other 

 by turning a handle. On the wood cylinder a spiral groove is 

 cut in which the wire lies, so that its successive convolutions 

 are insulated from each other ; but when any portion of the 

 wire is wound upon the brass cylinder, the current passes 

 immediately from the wire to the cylinder, and thence to one 

 of the terminals of the instrument. The effective part of the 

 length of the wire is therefore the variable portion which is 

 on the wood cylinder. 



In the other form there is only one cylinder, which is of 

 wood, and has a quantity of stout wire permanently wound 

 upon it in a spiral groove cut upon its surface. Near the 

 cylinder and parallel to its axis is a brass rod, upon which a 

 notched " rider/' or in more modern instruments a grooved 

 brass wheel, is capable of sliding. The notch in the rider, 

 or the groove in the wheel, fits and presses upon the spiral 

 wire, and when the cylinder is turned the rider or wheel 

 moves longitudinally along the brass rod. The terminals of 

 the instrument are connected respectively with the brass rod 

 and with one end of the wire upon the cylinder ; and the 

 resistance introduced into the circuit depends upon the posi- 

 tion of the point of contact of the rider or wheel with the wire. 



The apparatus in either, form is open to serious practical 

 objections. In that first described it is difficult to maintain 

 good electrical contact between the wire and the brass cylinder, 

 both of which must be kept perfectly clean and free from dust 

 and damp. Moreover the wire is liable to become slack and 

 to leave its groove, and not unfrequently it breaks. In the 

 second form there is also a difficulty in securing uniformly 

 good contact ; and if the slider fits the rod sufficiently tight, 

 the lateral pressure upon the wire is so great that it becomes 

 permanently stretched, and is sometimes forced completely out 

 of its groove. The wire, too, is necessarily thick ; and the whole 

 apparatus must consequently be large and cumbersome if it is 

 to work through any considerable range of resistance. 



A Wheatstone's rheostat in good working order is rarely 

 seen, even in the shop of the instrument maker ; and in point 

 of fact it is little used except for the purpose of lecture- 

 illustration. 



In the course of some experimental work the pressing need 

 of some means of continuously varying a resistance led me to 

 devise the modified rheostat, which is figured in the annexed 

 woodcut. 



As in the second form of Wheatstone's instrument, a wire 



