US3808874A

US3808874A - Contact tester

Info

Publication number: US3808874A
Application number: US00300920A
Authority: US
Inventors: T Trevithick
Original assignee: Daniel Woodhead Co
Current assignee: Woodhead Industries LLC
Priority date: 1972-10-26
Filing date: 1972-10-26
Publication date: 1974-05-07
Anticipated expiration: 1991-05-07
Also published as: CA995480A

Abstract

Contact tester for determining frictional resistance against withdrawal from female contact members of electrical connectors or receptacles by male blades or pins of plugs embodying a housing having one or two, independently spring-biased blades projecting from one end of its housing, a spring-biased pin projecting from the other end, and indicia scale means on the housing with a pointer moved by extension of each respective blade and the pin for reading frictional resistance by the receptacle''s female contacts against withdrawal of the blade or blades or the pin from the receptacle.

Description

United States Patent [191 [451 May 7, 1974 Trevithick CONTACT TESTER [75] Inventor: Tony Trevithick, Gumee, lll.

[73] Assignee: Daniel Woodhead Inc., Northbrook,

22 Filed: Oct. 26, 1972 [21] Appl. No.: 300,920

[52] U.S. Cl. 73/9, 177/232 51 Int. Cl. G0ln 19/02 [58] Field of Search 73/9; 177/232; 116/129 A, 116/129 B [56] References Cited UNITED STATES PATENTS 2,038,092 4/1936 Wheeler..; 73/9 2,549,190 4/1951 Gilchrist... 116/129 8 OTHER PUBLICATIONS Publ. Motors Auto Repair Manual by Motor Mag. 1953 (Hearst Corp.) pages l4, 16, 1'8, 21, 22. one end of its housing, a spring-biased pin projecting from the other Primary Examiner-Richard C. Queisser Assistant Examiner-Daniel M. Yashich Attorney, Agent, or Firm-Johnston, Keil, Thompson & Shurtleff [57] ABSTRACT Contact tester for determining frictional resistance against withdrawal from female contact members of electrical connectors or receptacles by male blades or pins of plugs embodying a housing having one or two, independently spring-biased blades projecting from one end of its housing, a spring-biased pin projecting from the other end, and indicia scale means on the housing with a pointer moved by extension of each respective blade and the pin for reading frictional resistance by the receptacles female contacts against withdrawal of the blade or blades or the pin from the receptacle.

10 Claims, 8 Drawing Figures CONTACT TESTER This invention concerns improvements in devices for determining frictional resistance to withdrawal of contact members of electrical receptacles against blades or pins of electrical plugs. Poor contact therebetween, even though current flows from the receptacle to the plug, may cause a voltage drop across the contacts, a heat build-up in the receptacle, arcing across contacts or other undesirable effects within the receptacle. The invention allows maintenance personnel, home owners, electricians and others to check quickly and conveniently the quality of receptacles, connectors and other electrical components having female contacts in terms of their contact pressure against the blades or pins of plugs and the like.

In certain electrical uses, good contact between male plug and female receptacle or connector contact components is critical. For example, in hospitals and laboratories, many instruments and machines depend for accuracy or proper functioning upon the precise supply voltage. Intermittentor constant voltage drops across poor contacts at the plug and receptacle can cause serious malfunctions in certain precise voltage-dependent instruments and machines.

One known type of contact tester comprises a cylindrical housing with a spring-biased metal rod extending into one end of acylindrical housing, the rod having a blade end projecting from the housing. A spring-biased pin projects from the other end of the housing. A scale with a window slot is used to read the positionof a member inside the housing which moves with the rod or pin as they extend from the housing upon withdrawal of the blade or pin from a receptacle opening. The user has to read the scale as he pulls the tester away from the receptacle because the member used for scale reading immediately snaps back to zero position under the spring pressure when the blade or pin loses its holding engagement with the receptacles contact member. Further, with the rod and pin both being metal, the user can be shocked accidentally, should he inadvertently touch directly, or indirectly through a conducting member, the portion of the rod which projects from the housing when he withdraws the tester while the blade is still in conducting contact with a live contact member of the receptacle.

THE INVENTION This invention concerns contact testers for measuring frictional resistance by female contact members of electrical receptacles or connectors against blades or pins of electrical plugs as the latter are withdrawn. The tester comprises a housing with one elongated, longitudinal cavity or a pair of elongated, longitudinal, sideby-side cavities in one end portion. A rod extends longitudinally slidably in the single cavity or both cavities. A first blade is connected with one rod and projects from an end of said housing. With two cavities, a second blade is connected with the other rod and projects from the same end of the housing. Spring means bias each rod in a direction from said end toward home position. The housing has in a wall thereof one or a pair of longitudinal slots opening into respective cavities with indicia scale means on the wall adjacent the single slot or pair of slots.

A scale pointer member is slidable along each slot. Each rod bears a member which respectively is engageable with a pointer member as the rod extends out of its cavity and is disengaged therefrom as the respective rod returns under the bias of the spring means to home position. Upon insertion of the single blade or both blades into receptacle or connector and withdrawal thereof, the frictional contact pressure of the respective contact members of the receptacle against the blades can be ascertained by noting the respective positions of the pointer members relative to the indicia scale means. 7

Each rod preferably is made of dielectric material to guard the user against electrical shock in case of accidental touching of the extended portions of the rod while the tester is being pulled away from the receptacle and its blade is still in contact with a live contact member of the receptacle. Each rod respectively bears a'projection on the end ofthe rod remote from the blade-projecting end of the receptacle. The projection is abuttingly engageable with a pointer member. Each pointer member may comprise a pointer head on the outside of the housing, a plate inside the housing extending across each slot, and a member extending through each slot and connecting the head and the plate. Each plate is in sliding, friction engagement with the inner wall of the housing adjacent its respective slot to hold the pointer head in the positions designating maximum extension of its rod when the blade or blades are withdrawn from said receptacle.

For a tester embodying a ground connection testing device, the housing has another longitudinal cavity in its opposite end portion. A pin or rod serving as a ground pin contact tester extends through the third cavity and projects from the opposite end of the housing. The pin is slidable in the cavity with spring means biasing the pin in a direction away from the opposite end toward home position. Another longitudinal slot in the housing opens into the third cavity. A follower pointer member is slidable in the latter slot. The outer side of the housing has additional indicia scale means adjacent the latter slot. A member on the pin is engageable with the latter follower as said pin extends out of the cavity and is disengageable therefrom as said pin returns under the bias of the spring means to home position. Upon insertion of the projecting end of the pin into the ground pin opening of an electrical receptacle or connector and withdrawal thereof, the frictional contact pressure of the ground pin-contact member of the receptacle against a ground pin can be ascertained by noting the position of the latter follower relative to the additional indicia scale means.

THE DRAWINGS Preferred embodiments of the invention are illus trated in the drawings, wherein:

FIG. 1 is a top plan view of a first embodiment of a contact tester; v

FIG. 2 and FIG. 3 are respective end elevations thereof as viewed from section lines 22 and 3-3 of FIG. 1;

FIG. 4 is a plan view of the interior of the lower section of the taken on section plane 4-4 of FIG. 2 tester; I

FIG. 5 is a plan view of the interior of the upper section of the tester;

FIG. 6 is a section view taken on section plane 6-6 of FIG. 4;

FIG. 7 is a section view taken on section plane 77 of FIG. and

FIG. 8 is a top plan view of another embodiment having a single blade instead of a pair of blades.

THE ILLUSTRATED EMBODIMENTS Referring to the drawings, the contact tester 10 comprises a two-section housing 11 having an upper section 12 and a lower section 13. The upper section 12 has a main body portion 14 and respective end walls 15 and 16. The body portion and end. walls preferably are a one piece molding from a thermoplastic or thermosetting polymer but may also be separate moldings, in which case their assembly isheld together by screws 17.

The lower section 13 likewise has a body portion 18 and

end walls

19 and 20, again preferably made as a one piece molding of a thermoplastic or thermosetting polymer. If made as separate moldings, screws 21 serve the same function as screws 17.

Each

blade

40 and 41 has a rear portion 42 embedded in the forward end of

respective rods

43 and 44. These rods, for safety reasons, are made of dielectric material, e.g., a hard rubber (synthetic or natural) or a thermosetting or thermoplastic polymer.

The

rods

43 and 44, each bearing a

respective blade

40 and 41 extend longitudinally through two, internal, side-by-side, longitudinal cavities formed in the housing 10. Each respective cavity comprises a

semicylindrical wall

45 and 46 formed in the lower section l3 and an opposing partially cylindrical, partially recti- The lower section 13 has longitudinal lips 22 along each side of its body portion. These lips matingly lie in longitudinal offsets or recesses 23 along each side of the upper section 12 when the sections are assembled. The assembly may be a press fit or a snap fit with the contacting surfaces of lips 22 and offsets or recesses 23 in tight engagement or the sections may be bonded by adhesive, solvent-welding of the plastic surfaces or the like.

The outer face 24 of the upper section 12 (FIG. 1) has a pairof substantially rectangular, recessed

walls

25 and 26, which have respective

longitudinal slots

28, 29 and 30. The latter two slots are parallel slots in the recessed wall 25.

lndicia scale means 31, 32 and 33 are provided on the contacttester. Each indicia scale means comprises a pointer head 34 and an indicia-bearing

scale

35 or 36 with markings readable in ounces. Scale 36 serves both indiciascale means 32 and 33.

The respective pointer heads 34 have a T-crosssection with a leg 37 extending through respective slots 28-30 (FIG. 7). The heads 34 will slide along their slots. Each head has a longitudinally bowed plate 38 attached to the base of its leg 37 by a screw 39. The ends of the bowed plate 38 bear against the inner face of

walls

26 or 27 on opposite sides of the respective slots in light, frictional contact with the inner face. Thelight frictional contact is sufficient to keep the pointer heads 34 in whatever position they are pushed along respective slots while still allowing them to slide along the slots when pushed. The

parts

34, 37 and 38 may be a one-piece molding of a thermoplastic polymer dimensioned to provide this light-frictional, sliding contact.

A pair of

blades

40 and 41 project from one end of the tester 10. These blades simulate the blades or I prongs of a male, electrical plug. Preferably they are metal blades dimensioned and oriented in accord with industry-accepted standards for the prongs or blades of electrical plugs. For purposes of illustration, the blades shown herein are dimensioned and oriented in one type of industry-accepted standard for 110-125 volt circuits with or without a ground contact. Other orientations of the locking and non-locking types with polarized or non-polarized blades and openings, e.g., according to standards set by the National Electrical Manufacturers Association (NEMA), may be used in lieu of that illustrated.

linear wall 47 and 48in the upper section 12. The cross-section of these cavities is shown in FIG. 6.

Each

rod

43 and 44 carries at its rearward, bladeremote end a disc 49 having an upper, rectangular ear or projection 50. The circular peripheral edge portion of each disc 49 is concentric with and slightly smaller than the circular cross-sectioned portions of the respective cavities, i.e., the portions formed by the

semicylindrical walls

45 and 46 and the partially cylindrical portion 51 of walls 47 and 48 (FIG. 6). Each rectilinear portion 52 of

walls

47 and 48 accommodates the ear or projection 50 of each disc 49. The

slots

29 and 30 in recessed wall 25 intersect and run longitudinally along most of the length of the rectilinear cavity formed by rectilinear portion 52 and the contiguous part of wall 25.-

Projecting from the other end of the contact tester 10 is a ground pin contact tester. It is a metal pin or rod 55 having its rearward portion 53 within a longitudinal cavity 54 configured the same as the previously described cavities. It bears on its rearward end a disc 49 with projection or ear 50.

The

respective blades

40 and 41 and their

rods

43 and 44 and the pin or rod 55 are spring-biased toward home position, i.e., their least extended positions, by respective coil or helix'springs-56. Each spring 56 is mounted about the

respective rods

43 and 44 and the rearward portion 53 of. rod or pin 55. One end of each spring bears against the disc 49 while the other end bears against an inner face of the

end walls

15, 20 and .16, 19 about the circular openings in the end walls through which the

rods

43 and 44 and the pin or rod 55 may be extended. Such circular openings conveniently are formed by semi-circular, opposed recesses 57, 58 and 59 provided in the abutting edges of the end walls'l6, 20 and 15, 19. The springs 56 are at most only lightly compressed when the

rods

43 and 44 and the pin or rod 55 are in their home positions, i.e., the positions shown in FIG. 4.

To test a receptacles contact pressure on the current conducting blades of an electrical plug and/or the ground pin thereof, the pointer heads 34 are placed at the zero scale reading.'The

blades

40 and 41 or the pin or rod 55 are inserted fully in the receptacle openings therefor. The tester is slowly withdrawn in a direction parallel with the

blades

40 and 41 or the rod or pin 55, i.e., at to the face of the receptacle. As it is withdrawn, a receptacle with good contact pressure will cause the

rods

43 and 44 of the

blades

40 and 41 or the rod or pin 55to extend from the housing 11. The discs 49 compress the helix springs 56 as they move with the rearward ends of the rods or pins forwardly through the respective longitudinal cavities toward the respective end walls of the housing. Simultaneously, the ear or projection 50 of each disc abuts against and pushes the bar 38 of each pointer head 34 thereby sliding it along the

respective slot

28, 29 and/or 30. This movement continues until the compressive pressure built up in each spring 56 overcomes the frictional resistance between

respective blades

40 and 41 or pin or rod 55 with the respective female contacts of the receptacle. At this point, the blade or pin slides out of the receptacle and the blade-rod 4t), 43 or the blade-

rod

41, 44, or the pin or rod 55, snaps under pressure of respective springs 56 back to its least fully extended, home position. The pointer head 34, however, remains at the place on the scale corresponding to the most fully extended position to provide a reading of the maximum resistance against blade or pin withdrawal by the receptacles contact members by the testing personnel after complete withdrawal of the contact tester from the receptacle.

Each receptacle contacts resistance to withdrawal against

blades

40 and 41 is measured independently of the other. FIG. ll illustrates what might be a reading on each scale for a receptacle or connector tested by the tester of this invention. The resistance to withdrawal of blade 40 is weak and suggestive of need for repair or replacement of the contact orthe entire receptacle. The higher withdrawal resistance readings for blade 41 and pin 55 for most cases would be judged to be satisfactory contact performance.

As stated aforesaid,

rods

43 and 44 are made of dielectric material. During the testing procedure, the

conductive blades

40 and 41 remain in their fully inserted position in the receptacle until the blade and its rod snap back to home position. The rod, however, extends progressively from the housing until the snap back occurs. By making it of dielectric material, the possibility of accidental shock is avoided. Such shock could occur if the rod were conductive, through touching of the rod, directly or indirectly, while the blade is still in conductive contact with the receptacles contact member. Thus, there is no need for safety reasons to disconnect or open the receptacles circuit during the test procedure.

The embodiment of FIG. 8 differs from the first embodiment described above only in that it has a single blade and associated rod, longitudinalcavity, etc. This contact tester 60 has a ground pin-testing rod or pin 55 projecting from one end as in the previous embodiment and only a single blade 61 projecting from the other end. The latter is simply one of the

blades

40 or 41 with its

respective rod

43 or 44, spring 56, disc 49 and its ear 50 in a cavity having the cross-section shown in FIG. 6. Like numerals in the first embodiment designate like parts in FIG. 8.

The embodiment of FIG. 8 has universal application for testing contact members of receptacles or connectors of any geometric arrangement on the face of the receptacle or connector. Each contact member is tested sequentially with the reading on indicia scale 36 noted after withdrawal of blade 61 from each contact member.

it is thought that the invention and its numerous attendant advantages will be fully understood from the foregoing description, and it is obvious that numerous changes may be made in the form, construction and arrangement of the several parts without departing from the spirit or scope of the invention, or sacrificing any of its attendant advantages, the forms herein disclosed trating the invention.

The invention is hereby claimed as.follows:

pressure of contact members of an electrical receptacle or connector against blades or pins of electrical plugs inserted therein comprising a housing, an elongated, longitudinal cavity in said housing, a rod extending slidably longitudinally in said cavity, said rod bearing a blade projecting from an end of said housing, spring means biasing said rod in a direction from said end toward its least-extended, home position, a longitudinal slot in said housing opening into said cavity, indicia scale means adjacent said slot, a follower slidable in said slot, means on said rod engageable with said follower and moving said follower in said slot toward said end as said rod extends out of its cavity toward said end and disengageable therefrom as said rod returns under the bias of said spring means to said home position, whereby, upon insertion of said blade into a receptacle or connector and withdrawal thereof, the linear movement of said rod bearing said blade resulting from the frictional contact pressure of the contact member of the receptacle or connector against said blade during the withdrawal of the blade can be ascertained by noting the position of said follower relative to said indicia scale means.

2. A contact tester as claimed in claim 1, said rod being dielectric to guard the user against electrical shock in case of accidental touching of the extended portions of said rod while the tester is being pulled away from said receptacle or connector and said blade is still in contact with a live contact member of said receptacle or connector.

3. A contact tester as claimed in claim 1, said means on said rod comprising a projection on the end of the rod remote from said end of said housing and abuttingly engageable with said follower.

4. A contact tester as claimed in claim 3, said follower comprising a pointer head outside said housing, a member inside said housing extending across said slot, and a leg extending through each slot and connecting said head and said member.

5. A contact tester as claimed in claim 4, said member being in sliding, friction engagement with the inner wall of ,said housing adjacent the slot to hold said pointer head in the position of maximum extension of said blade and rod when said blade is withdrawn from said receptacle or connector.

6. A contact tester as claimed in claim 1, said follower comprising a pointer head outside said housing, a member inside said housing extending across said slot, and a leg extending through said slot and connecting said head and said member.

7. A contact tester as claimed in claim 6, said member being in sliding, friction engagement with the inner wall of said housing adjacent said slot to hold said pointer head in the position of maximum extension of said blade and rod when said blade is withdrawn from said receptacle or connector.

8. A contact tester as claimed in claim 1, a further longitudinal cavity in said housing, a pin serving as a ground pin contact tester extending through said further cavity and projecting from said housing, said pin being slidable in said cavity, further spring means biasing said pin toward its least-extended, home position, a further longitudinal slot in said housing opening into said further cavity, a'follower slidable in said further slot, indicia scale means adjacent said further slot, and

means on said pin engageable with said last-mentioned follower as said pin moves out of said further cavity and disengageable therefrom as said pin returns under the bias of said further spring means to home position,

whereby upon insertion of the projecting end of said pin into the ground pin opening of an electrical receptacle or connector and withdrawal thereof, the frictional contact pressure of the ground pin-contact member of the receptacle or connector against the ground pin can be ascertained by noting the position of said last-mentioned follower relative to said indicia scale means adjacent said further slot.

9. A contact tester for determining frictional contact pressure of female contact members of electrical receptacles against male blades or pins of electrical plugs inserted therein comprising a housing, a pair of elongated, longitudinal, side-by-side cavities in said housing, a rod extending slidably longitudinally in each cavity, one rod bearing a first blade projecting from an end of said housing, the other rod bearing a second blade projecting from said end of said housing, spring means biasing said rods in a direction from said end toward home position, a pair of longitudinal slots in said housing opening into respective cavities, indicia scale means adjacent said slots, a follower slidable in each slot, means on each of said rods respectively engageable with a follower and moving said follower in said slot toward said end as said rods extend out of their cavities toward said end and disengageable therefrom as said rodsreturn under the bias of said spring means to home position, whereby, upon insertion of said blades into a receptacle and withdrawal thereof, the linear movement of said rods bearing their respective blades resulting from the frictional contact pressure of the respective contact members of the receptacle against respective blades during the withdrawal of said blades can be ascertained by noting the respective positions of said followers relative to said indicia scale means.

10. A contact tester as claimed in claim 9, a longitudinal third cavity in said housing, a pin serving as a ground pin contact tester extending through said third cavity and projecting from said housing, said pin being slidable in said third cavity, further spring means biasing said pin toward its least-extended, home position, a third longitudinal slot in said housing opening into said third cavity, a third follower slidable in said third slot, indicia scale means adjacent said third slot, and means on said pin engageable with said third follower as said pin moves out of said third cavity and disengageable therefrom as said pin returns under the bias of said further spring means to home position, whereby upon insertion of the projecting end of said pin into the ground pin opening of an electrical receptacle and withdrawal thereof, the frictional contact pressure of the ground pin-contact member of the receptacle against the ground pin can be ascertained by noting the position of said third follower relative to said indicia scale means ',UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No. 3,808n874 Dated May 7, 1974 Tony Trevithick Inventor(s) It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 63, "taken on section plane 4-4 of Fig. 2

tester" should read tester taken on section plane 4-4 of Figure 2 Signed and sealed this 15th day of October 19740 (SEAL) Attest: I McCOY M. GIBSON JR. c. MARSHALL DANN Commissioner of Patents Attesting Officer USCOMM-DC 80376-P59 U Sv GOVERNMENT PRINTING OFFICE: 8 9 9 0 FORM P Q-1050 (10-69) QUNITEDIQ STATES PATENT OFFICE 'CER TIFICATE OF CORRECTION v 3,808 874 Dated May 7, 1974 Patent No.

Invent-(8) Tony Trevlthlck It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 2, line 63, 'tetken on section plane 4-4 of Fig. 2

tester" should read tester taken on section plane 4-4 of Figure 2 Signed and sealed this 15th day of October 1974 (SEAL) Attest: I McCOY M. GIBSON JR. c. MARSHALL DANN Commissioner of Patents Attesting Officer USCOMM-DC 60376-P69 U.S GOVERNMENT PRINTING O FICE: 869- 9 O F ORM PC4050 (10-69)

Claims

1. A contact tester for determining frictional contact pressure of contact members of an electrical receptacle or connector against blades or pins of electrical plugs inserted therein comprising a housing, an elongated, longitudinal cavity in said housing, a rod extending slidably longitudinally in said cavity, said rod bearing a blade projecting from an end of said housing, spring means biasing said rod in a direction from said end toward its least-extended, home position, a longitudinal slot in said housing opening into said cavity, indicia scale means adjacent said slot, a follower slidable in said slot, means on said rod engageable with said follower and moving said follower in said slot toward said end as said rod extends out of its cavity toward said end and disengageable therefrom as said rod returns under the bias of said spring means to said home position, whereby, upon insertion of said blade into a receptacle or connector and withdrawal thereof, the linear movement of said rod bearing said blade resulting from the frictional contact pressure of the contact member of the receptacle or connector against said blade during the withdrawal of the blade can be ascertained by noting the position of said follower relative to said indicia scale means.

5. A contact tester as claimed in claim 4, said member being in sliding, friction engagement with the inner wall of said housing adjacent the slot to hold said pointer head in the position of maximum extension of said blade and rod when said blade is withdrawn from said receptacle or connector.

8. A contact tester as claimed in claim 1, a further longitudinal cavity in said housing, a pin serving as a ground pin contact tester extending through said further cavity and projecting from said housing, said pin being slidable in said cavity, further spring means biasing said pin toward its least-extended, home position, a further longitudinal slot in said housing opening into said further cavity, a follower slidable in said further slot, indicia scale means adjacent said further slot, and means on said pin engageable with said last-mentioned follower as said pin moves out of said further cavity and disengageable therefrom as said pin returns under the bias of said further spring means to home position, whereby upon insertion of the projecting end of said pin into the ground pin opening of an electrical receptacle or connector and withdrawal thereof, the frictional contact pressure of the ground pin-contact member of the receptacle or connector against the ground pin can be ascertained by noting the position of said last-mentioned follower relative to said indicia scale means adjacent said further slot.

9. A contact tester for determining frictional contact pressure of female contact members of electrical receptacles against male blades or pins of electrical plugs inserted therein comprising a housing, a pair of elongated, longitudinal, side-by-side cavities in said housing, a rod extending slidably longitudinally in each cavity, one rod bearing a first blade projecting from an end of said housing, the other rod bearing a second blade projecting from said end of said housing, spring means biasing said rods in a direction from said end toward home position, a pair of longitudinal slots in said housing opening into respective cavities, indicia scale means adjacent said slots, a follower slidable in each slot, means on each of said rods respectively engageable with a follower and moving said follower in said slot toward said end as said rods extend out of their cavities toward said end and disengageable therefrom as said rods return under the bias of said spring means to home position, whereby, upon insertion of said blades into a receptacle and withdrawal thereof, the linear movement of said rods bearing their respective blades resulting from the frictional contact pressure of the respective contact members of the receptacle against respective blades during the withdrawal of said blades can be ascertained by noting the respective positions of said followers relative to said indicia scale means.

10. A contact tester as claimed in claim 9, a longitudinal third cavity in said housing, a pin serving as a ground pin contact tester extending through said third cavity and projecting from said housing, said pin being slidable in said third cavity, further spring means biasing said pin toward its least-extended, home position, a third longitudinal slot in said housing opening into said third cavity, A third follower slidable in said third slot, indicia scale means adjacent said third slot, and means on said pin engageable with said third follower as said pin moves out of said third cavity and disengageable therefrom as said pin returns under the bias of said further spring means to home position, whereby upon insertion of the projecting end of said pin into the ground pin opening of an electrical receptacle and withdrawal thereof, the frictional contact pressure of the ground pin-contact member of the receptacle against the ground pin can be ascertained by noting the position of said third follower relative to said indicia scale means.