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Adjustable Gap Transfer

Technical Information

Catalogue No: C0148
Category: Unknown
Object Type: Test Equipment
Object Name: Adjustable Gap Transfer
Part No: None
Serial No: None
Manufacturer: Unknown
Division: Unknown
Year of Manufacture: circa 1955
Width (mm):
Height (mm):
Depth (mm):
Weight (g):
Location: Unknown



The Adjustable Gap Transfer is a current clamp or current probe which is an electrical device having two jaws which open to allow clamping around an electrical conductor. These devices have a split ring made of ferrite or soft iron. A wire coil is wound round one or both halves, forming one winding of a current transformer. The conductor around which it is clamped forms the other winding. Like any transformer this type works only with AC or pulse waveforms.
The instrument gives a safe and remote measure of the current passing through the cable. It is marked on a label as having three ranges 50, 10.5 and 2.5 Amp. There is also a note that it is ‘Pre-Shotter Patent. But not patented’. George Frederick Shotter was quite a prolific inventor working at New Southgate in England and he has many Patents relating to transformers and motor design. Some of his Patents relevant to this Adjustable Gap Transfer can be found around 1926 but this is more likely to be an experimental device from the 1950 -60 period.

This item was part of the original Elliott Collection Ref: 1018

Pincer construction, 18 cms overall length. The two arms compress against a coil spring thus opening to clip round a cable or busbar conductor. The pincer heads carry insulated coils with two flexible flying leads, one red/white, one blue/white, to spade connectors (one missing). The gap in the laminated core is controlled by a screw through one of the pincer heads. An indicator is visible through a small circular glazed aperture and shows small circles of yellow, green and red as the screw is turned. The '50' refers to the frequency but has been misread as a current in amperes. The finish appears to be black enamel.

A current transformer is used with an ammeter or power meter to obtain measurements of the alternating current in a conducting cable or busbar. Versions may be connected permanently or arranged in a clip-on form, such as the present example. Current transformers , particularly the clip ' on type, normally find application where it is undesirable to break into a conductor where large currents or voltages are present, or where continuous measurements are not required.
A current transformer is calibrated for use with a particular ammeter or wattmeter. The current in the coil of the instrument transformer is dependent on the turns ratio as in a normal transformer, but also on other factors. The calibration is affected if the meter or a range on the meter is changed. This adjustable gap transformer appears to be intended to allow range changing by adjusting the magnetic circuit (by means of the screw provided) without affecting the calibration of the meter in use.
The note 'pre Shotter patent' suggests that the idea was conceived by Shotter who was a highly creative engineer at Elliott Bros., but possibly superseded by a further development. Collection item 1017 is a current transformer to a Shotter patent: a search of this and other Shotter patents might reveal more information about both items.

Numerous, esp. Golding, 'Electrical Measurements and Measuring Instruments' Pitman 1961 edition

H.R.Bristow 5 August 2010

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