RichardsRadios

Description of EVERSHED & VIGNOLES Portable Testing Set [1900 pattern]

[The following has been reproduced from a copy of E&V List No.2 dated March 1904 provided by C M Deavin who gave me copies of a number of original Evershed and Vignoles documentation provided by Marion Heard, the publicity assistant at Megger back in 2003.]


EVERSHED & VIGNOLES Ltd.


EVERSHED'S PORTABLE TESTING SET [1900 pattern]
The Evershed Testing Set consists of a direct reading Ohmmeter, which indicates by a simple scale reading, the value of the resistance being tested, and a portable hand dynamo or Generator which provides the testing current at any required pressure. This set has now come into universal use; its simplicity, convenience, and accuracy having secured its adoption by nearly all supply stations, wiring contractors and dynamo builders.

OHMMETER
In the pattern of Testing Set now sold the Ohmmeter is an improved form of that introduced in 1895, a pattern which has proved in every respect satisfactory. This Ohmmeter was specially designed with the object of avoiding liability to damage through careless use. The astatic soft iron needle is magnetised by the current from the Generator, rendering the Ohmmeter reading independent of the direction of the testing current; and reversal of the connections between the instruments, or of the direction of rotation of the Generator, docs not injure the Ohmmeter in any way nor affect the accuracy of its reading.

GENERATOR
The Generator has been entirely remodelled, the 1900 pattern being a great improvement upon the earlier forms both as regards mechanical and circuit design. An important advantage has been gained in the fact that the new construction enables a higher voltage to be obtained with safety, and it will be noticed that a 1,000 volt Generator has now been added to our list. The weight of the Generator is less than that of the old pattern, and the effort required to drive it has been largely reduced by the introduction of several novel features; the armature coils arc wound in holes on the periphery of a laminated core made of the best quality of "transformer" iron, thus reducing the hysteresis and eddy current losses to a minimum. The collecting brushes are in the form or elastic discs rolling in contact with the ring segments of the commutator, a device which enormously lessens the friction commonly met with when fixed brushes are used and at the same time ensures perfect contact at all speeds---a point of great importance in a dynamo for testing purposes. The gearing between. the handle and the armature axle is cut from solid rings of compressed wrought brass and runs with perfect smoothness. The handle is fixed on its axle, and held in position for turning by means of a spring catch. When not in use it may be turned back into a recess in the wood case. The standard patterns of the 1900 Generator are wound for 100 volts, 200 volts, 500 volts, and 1,000 volts.

The 1900 Pattern Generator can be supplied fitted with a spring drum inside the case, on which a twin flexible cord is coiled, provided with a connector adapted for clamping under the Ohmmeter terminals. This attachment is very convenient and is strongly recommended, as it obviates the necessity for carrying separate leads. It is shown in use in the block which illustrates this List. Generators not provided with spring drums have their terminals outside the case.

PRECAUTIONS TO BE OBSERVED IN USING THE TESTING SET
When making tests where there are likely to be stray magnetic fields---for example near dynamos, or mains carrying large currents---readings should be taken with the Generator handle turned first in one direction and then in the other. If the stray field produces any error, one reading will be as much too high as the other is too low, and the average of the two readings will be the true value of the resistance under test.

An Ohmmeter can only measure resistance when the Generator supplying the testing current is perfectly insulated. Hence the use of the volts on the Street mains leads to entirely fallacious result, since leaks invariably exist on the mains or on the dynamos supplying them. For the same reason it is not sufficient when testing wiring to remove the fuses at the switchboard, couple the Ohmmeter to the blocks on the consumer's side, and then to make insulation tests while the station volts are on the fuse blocks on the supply side. Current will leak from the supply blocks into the consumer's circuits, and the tests will be worthless. Each circuit must be removed from its fuse blocks and coupled directly to the Ohmmeter for testing; or better still the street mains should be cut off by a well insulated double pole switch. If with all possible precautions leakage on the supply cannot be entirely avoided, two readings should be taken while turning the Generator handle in reverse directions, precisely as described above for eliminating errors due to stray fields. If the leakage is small, the average of the two readings will give the true value of the resistance under test.

EVERSHED & VIGNOLES, Ltd. PORTABLE TESTING SETS.


PORTABLE TESTING SET.
(EVERSHED'S PATENT.)

INSTRUCTIONS FOR USE

Adjust the Ohmmeter until the bubble is in the centre of the spirit level.
Place the Generator not less than 18 inches away from the Ohmmeter, and couple its terminals to the terminals marked G, G on the Ohmmeter.
Couple the mains to be tested to the "LINE" and "EARTH " terminals of the Ohmmeter. Turn the Generator handle steadily in either direction at any speed above 60 revolutions per minute, and the Ohmmeter index will point to the resistance under test.


Max ohmmeter reading

Generator voltage

Price

Extra for spring drum

5MΩ

100V

18

15s

10MΩ

200V

20

15s

50MΩ

500V

25

15s

50MΩ

1000V

30

15s

100Ω *

10V

20

not made

*other ranges can be supplied



These were very expensive instruments. To put this in perspective, in 1900 male teachers earned around 150- 200 per annum and a three bedroom terrace house would have cost between 200 and 300.