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In some applications, such as dynamo-powered bicycle lights or torches (the old-fashioned ones with a metal barrel) the mass is truely integrated in the circuit. That is, the wiring does not form a closed circuit with reference to the mass, but the metal of the housing is used as conductor for considerable lengths. See also the circuit in the right below (taken from fig. 2b in the answer to this question) where the volt-meter could have been a lamp or anything else.

enter image description here

Is this good practice, or advisable only in such exceedingly simple circuits? Is it safe to have stronge currents freely floating through the mass? What considerations exist not to do this?

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    \$\begingroup\$ I'm assuming by "mass" you mean the chassis of the device. The simplest problem with this is that of conductivity. What's your chassis made of, and how does its conductivity compare to copper? \$\endgroup\$ – Hearth Mar 14 '19 at 18:30
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    \$\begingroup\$ @user508402: Please add your location into your user profile. It helps when we are trying to understand odd words or strange phrases in questions and answers. Hearth is correct: you probably mean "chassis" or "frame". \$\endgroup\$ – Transistor Mar 14 '19 at 18:32
  • \$\begingroup\$ I think with "mass" he is referring to "ground". \$\endgroup\$ – jusaca Mar 14 '19 at 18:42
  • \$\begingroup\$ From the linked question, the comments have indicated that "mass" was a mis-translation of the German word "Masse," called "ground" in English. Please use "ground" if that's what you really mean. US (and possibly other) users: "torch" is the non-North-American way of saying "flashlight." Wikipedia source \$\endgroup\$ – Kevin Kruse Mar 14 '19 at 18:46
  • \$\begingroup\$ "Ground" would be a poor choice of words on a moving vehicle. See my answer. \$\endgroup\$ – Transistor Mar 14 '19 at 18:46
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The illustration you posted is mine and (a) was for a different purpose but (b) suits your question.

Is this good practice, or advisable only in such exceedingly simple circuits?

Yes. Most of the worlds cars, vans, trucks and buses use the metal chassis of the vehicle as the common return conductor for the DC circuits.

Is it safe to have strong currents freely floating through the mass? What considerations exist not to do this?

At low voltages it is generally quite safe. The amount of metal available in the chassis of a car provides adequately low resistance despite steel's higher resistance when compared to copper. At higher voltages you need to follow local regulations and perform a risk analysis under possible fault conditions.

schematic

simulate this circuit – Schematic created using CircuitLab

Figure 1. A simple car lamp circuit using the chassis as a return conductor.

Note that the chassis ground symbol is preferred in this case to avoid confusion with the earth symbol (shown in your question) as the latter implies a connection with Earth which normally is not the case on a vehicle.

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