## 1 Introduction  
In a recent publication, [PERSON] [1] initiated a discussion on the definition of arc length in gas metal arc welding (GMAW) and the relationship between arc length and welding arc voltage. The author pointed out that the arc length--defined as the distance from the tip of the welding electrode to the adjacent surface of the weld pool [2; 3]--is obsolete and meaningless. This is especially apparent when one considers the whole range of arc welding processes that are currently available, such as advanced pulsed gas metal arc welding (GMAW-P) processes, controlled metal transfer [4; 5], and some of the transfer modes observed in constant voltage GMAW such as streaming and rotating spray [6; 7]. One of the difficulties associated with establishing a definition or reference standard is related to the precise identification of the electrode tip and the surface of the weld pool, as in the case of buried arc, making it impractical to determine an accurate measurement of this distance. The difficulties related to wire tip identification can lead to misleading results if the imaging technique is not appropriate. According to [PERSON] [1], a more consistent definition of arc length would be to consider the distance between the tip of the wire electrode taper to the surface of the weld pool, in which case it would be applicable for both GMAW and GMAW-P. This definition, along with being more general, is physically more consistent, and agrees with the results of [PERSON] et al. [8; 9] which show that depending on the amount of iron metal vapor present in the arc, less than 20% of the instantaneous current is flowing through the electrode tip. This is due to the shift in arc attachment position to the tapered region caused by the presence of a high concentration of iron vapor at the wire tip [10; 11; 12; 13; 14]. Therefore, the definition that the electric arc comprises the region between the electrode tip and the surface of the weld pool is not quite physically accurate. Another question that arises from the current definition of arc length [2; 3] is the statement of proportionality between arc voltage and arc length, well known to welders and even researchers actively working in the field of the science and technology of welding. Even if a precise arc length is measured according to the aforementioned standards, this statement is not accurate when comparing the voltage measured when transfer modes such as projected, streaming, and rotating spray [6; 7; 12; 15; 16; 17] or advanced pulsed GMAW [1] are considered. This has been based on high-speed imaging and high-speed data acquisition of the welding electrical signals. The need to accurately describe arc length has practical implications for the development of parameters for welding procedures, and this work reveals the limitations of direct visual observation. However, regardless of the actual arc length, welding parameters and procedures will remain self-consistent, since they rely on contact tip to work piece distance, and the actual length is of greater relevance to research studies. According to the literature, the structure of the electric arc in GMAW assumes a conical shape, and is divided in two distinct regions: an outer cone composed mostly of the ionized shielding gas, and an inner cone, which presents a high concentration of electrode metal vapor [8; 9; 10; 11; 12; 13; 18; 19; 20; 21; 22]. This arc structure has been reported via spectroscopy measurements [18; 19; 20; 21; 23], numerical simulations [8; 9; 24], and high-speed imaging experiments [25]. This last technique requires appropriate apparatus and settings to be employed in order to reveal the details of the different arc regions, metal transfer, and future formation, in order to provide useful information regarding the welding process and metal transfer phenomenon.