ON AN ELECTRIC CUfiEENT METEK. 565 



received the several names of ' secondary generator,' ' transformer,' and 

 ' converter.' This is the only thoroughly worked out system available to 

 the engineer for an extensive supply of electricity. Currents of an alter- 

 nating character (i.e., alternately positive and negative in direction, the 

 alternations being at the rate of some hundreds per second of time), and 

 of high tension or pressure, are by this system carried from the engine- 

 house, by comparatively thin and cheap wire conductors, to the points of 

 supply. The only diflficulty which has been met is in the designing of a 

 suitable meter. There is absolutely no meter available that pretends to 

 be reliable. The very best indicates a totally different result when the 

 same current is passed through it, if the number of alternations of the 

 current {i.e., the speed of the dynamo) be altered. 



It was to overcome this source of trouble and to remove the last diffi- 

 culty from an otherwise perfect system of electric distribution that the 

 author undertook the labour of designing and perfecting the meter here 

 described. Some idea of the work expended in bringing it to its present 

 state of perfection will be gained when it is stated that the trial observa- 

 tions during the development of the instrument number nearly 10,000. 



Seeing that the only electrical actions available were those of chemical 

 action, electro-magnetic action, and heat ; that the chemical method is 

 incapable of being used with alternate currents ; and that all electro- 

 magnetic meters must vary in their indications with the rapidity of the 

 alternations, the author was led to base his instrument on the heat 

 developed by an electric current. Such an instrument must be equally 

 applicable to continuous currents and to alternate currents, whatever 

 their rate of alternations. Thus a meter is obtained which is practically 

 perfect, and more simple in construction than any of those designed for 

 a more limited range of uses. 



The instrument is extremely simple both in principle and in con- 

 struction. It consists essentially of a flat spiral of iron wire with two 

 terminals. Sometimes these two terminals are united to one wire, the 

 other being attached to the middle of the iron wire. Thus the instrument 

 exhibited may be used as an accurate measure for currents from half an 

 ampere or from one ampere upwards. 



Above the conductor a set of vanes is pivoted. This consists of 

 a circular disc of mica with a hole in the centre in which is fixed a 

 paper cone carrying at its apex a pinion with a concentric ruby cup. 

 Round the circumference of the mica disc eight small cylinders of pith 

 are fixed at equal distances, and eight vanes inclined at '45° to the mica 

 disc are attached to the pith cylinders, these vanes being made of the 

 thinnest mica. This set of vanes is supported by the ruby cup resting 

 on a steel point fixed to the base of the instrument. The pinion engages 

 with the first wheel of a train of wheelwork actuating the indexes, 

 which show upon two dials the number of revolutions made by the 

 vanes. 



The action of the instrument is very simple. The electric current 

 passing through the iron conductor creates heat, which sets up a con- 

 vection current in the air, and tbis causes the vanes to rotate about 

 the vertical axis and drive the clockwork. The number of revolutions 

 indicated on the dials is, through a considerable range of currents, an 

 exact indication of the number of coulombs or ampere-hours which have 

 passed through the conductor. The friction of the ruby cup on the 



