Machine screw crew threads when torqued correctly are gas tight. The problem is the clamping action produced by the box fastening from one box to another is NOT gas tight unless you use a lock washer under the screw head fastening the boxes together. The screw is reliably grounded by the machine threads, the screw head by conduction to the threads, but the contact from the tab of the next box under the screw head is not as good. Luckily there is a lot of area, and the connection remains low enough impedance to do the job at 120 V. In a dry interior wall location the grounding to the zinc plated box remains good. Excessive dampness over many years can cause enough corrosion of the box surface the gangs do not remain well bonded. I have seen enough of these in bathrooms (especially) and in damp outdoor locations (porch ceilings and protected porch walls not requiring wet treatment) over the years to now pigtail the boxes together and not rely solely on the clamp screws. When a digital meter says open, it really means open. It is just one more pigtail per section for ganged boxes in damp locations. Dry interior locations, I just rely on the ganging screws. This post was edited by brickeyee on Thu, Apr 18, 13 at 13:03

This is a common 'trick' for large loads like motors. By bringing out more of the motor windings from the stator the device can be used at more than one operating voltage. It increases the complexity of the windings though since they must be divided into appropriate sections and then the terminals made available. The same thing s done with motors designed for 120/240 V operation (not "high voltage" at all). It has littler to do with branch circuit wiring topology. You will notice we do not have 120 V bulbs and 60 V bulbs. This is what would be required to have some portion of alighting circuit in series and some portion in parallel. I personally do not do production wiring and use crimps for much at all. You will learn to dislike them when you start doing remodeling work and are no longer doing all 'new work'. I do charge extra when I am forced to deal with remodeling that has crimp connections. Simply cutting them off is often not an option since the remaining conductor length often then violates the minimum length required from the face of the junction box. I am sure it speeds up the 'production' installers though. Right now I have a house with a built in on one side of a wall, and a tile backsplash on the other in a kitchen. I need a couple new circuits for under-cabinet lighting. It looks like I will have to go through the back of the kitchen cabinets and fish upwards to the new junction boxes in the back of the upper cabinets. At least there are no crimps present in the boxes already containing some lighting switches.

I'm confused. You purchased a new what? Older lamps typically didn't have a ground but some had a polarized plug so that the exposed metal was at least connected to the grounded conductor. Now if you want to connect a ground, you have to be careful. If the current carrying wiring is connected to the exposed metal, you need to leave the ground alone. If it's connected only to the bulb sockets (typically the threaded part not the button in the middle) then the rest of the metal should be grounded.

2008 NEC 250.130 Equipment Grounding Conductor Connections. Equipment grounding conductor connections at the source of separately derived systems shall be made in accordance with 250.30(A)(1). Equipment grounding conductor connections at service equipment shall be made as indicated in 250.130(A) or (B). For replacement of non– grounding-type receptacles with grounding-type receptacles and for branch-circuit extensions only in existing installations that do not have an equipment grounding conductor in the branch circuit, connections shall be equipment grounding conductor in the branch circuit, connections shall be permitted as indicated in 250.130(C) (C) Nongrounding Receptacle Replacement or Branch Circuit Extensions. The equipment grounding conductor of a grounding-type receptacle or a branch-circuit extension shall be permitted to be connected to any of the following: (1) Any accessible point on the grounding electrode system as described in 250.50 (2) Any accessible point on the grounding electrode conductor (3) The equipment grounding terminal bar within the enclosure where the branch circuit for the receptacle or branch circuit originates (4) For grounded systems, the grounded service conductor within the service equipment enclosure (5) For ungrounded systems, the grounding terminal bar within the service equipment enclosure FPN: See 406.3(D) for the use of a ground-fault circuitinterrupting type of receptacle. 406.3 General Installation Requirements. Receptacle outlets shall be located in branch circuits in accordance with Part III of Article 210. General installation requirements shall be in accordance with 406.3(A) through (F). (D) Replacements. Replacement of receptacles shall comply with 406.3(D)(1), (D)(2), and (D)(3) as applicable. (1) Grounding-Type Receptacles. Where a grounding means exists in the receptacle enclosure or an equipment grounding conductor is installed in accordance with 250.130(C), grounding-type receptacles shall be used and shall be connected to the equipment grounding conductor in accordance with 406.3(C) or 250.130(C). (2) Ground-Fault Circuit Interrupters. Ground-fault circuit-interrupter protected receptacles shall be provided where replacements are made at receptacle outlets that are required to be so protected elsewhere in this Code. (3) Non–Grounding-Type Receptacles. Where attachment to an equipment grounding conductor does not exist in the receptacle enclosure, the installation shall comply with (D)(3)(a), (D)(3)(b), or (D)(3)(c). (a) A non–grounding-type receptacle(s) shall be permitted to be replaced with another non–grounding-type receptacle(s). (b) A non–grounding-type receptacle(s) shall be permitted to be replaced with a ground-fault circuit interruptertype of receptacle(s). These receptacles shall be marked "No Equipment Ground." An equipment grounding conductor shall not be connected from the ground-fault circuitinterrupter-type receptacle to any outlet supplied from the ground-fault circuit-interrupter receptacle. (c) A non–grounding-type receptacle(s) shall be permitted to be replaced with a grounding-type receptacle(s) where supplied through a ground-fault circuit interrupter. Grounding-type receptacles supplied through the groundfault circuit interrupter shall be marked "GFCI Protected" and "No Equipment Ground." An equipment grounding conductor shall not be connected between the groundingtype receptacles.