reusable devices can come in a kit that contains a battery, a charger, and at least one cartridge. varying nicotine concentrations are available and nicotine delivery to the user also varies based on different cartomizers, e-liquid mixtures, and power supplied by the battery. these manufacturing differences affect the way e-cigarettes convert the liquid solution to an aerosol, and thus the levels of ingredients, that are delivered to the user and the surrounding air for any given liquid. first-generation e-cigarettes use lower voltages, around 3.7 v. second-generation second-generation pv. second generation devices started in 2013, and tend to be used by people with more experience. they are larger overall and look less like tobacco cigarettes. they usually consist of two sections, basically a tank and a separate battery. their batteries have higher capacity, and are not removable. being rechargeable, they use a usb charger that attaches to the battery with a threaded connector. certain batteries have a "passthrough" feature so they can be used even while they are charging. second-generation e-cigarettes commonly use a tank or a "clearomizer". clearomizer tanks are meant to be refilled with e-liquid, while cartomizers are not. because they're refillable and the battery is rechargeable, their cost of operation is lower. they can also use cartomizers, which are pre-filled only. some cheaper battery sections use a microphone that detects the turbulence of the air passing through to activate the device when the user inhales. other batteries like the ego style can use an integrated circuit, as well as a button for manual activation. the led shows battery status. the power button can also switch off the battery so it is not activated accidentally. second generation e-cigarettes may have lower voltages, around 3.7 v. adjustable-voltage devices can be set between 3 v and 6 v. third-generation third-generation pv. the third-generation started in 2013, it included mechanical mods and variable voltage devices. battery sections are commonly called "mods," referencing their past when user modification was common. mechanical mods do not contain integrated circuits. they are commonly cylindrical or box-shaped, and typical housing materials are wood, aluminium, stainless steel, or brass. a larger "box mod" can hold bigger and sometimes multiple batteries. mechanical mods and variable devices use larger batteries than those found in previous generations. common battery sizes used are 18350, 18490, 18500 and 18650. the battery is often removable, so it can be changed when depleted. the battery must be removed and charged externally. variable devices permit setting wattage, voltage, or both. these often have a usb connector for recharging; some can be used while charging, called a "passthrough" feature. the power section may include additional options such as screen readout, support for a wide range of internal batteries, and compatibility with different types of atomizers. third-generation devices can have rebuildable atomizers with different wicking materials. these rebuildables use handmade coils that can be installed in the atomizer to increase vapor production. hardware in this generation is sometimes modified to increase power or flavor. the larger battery sections used also allow larger tanks to be attached that can hold more e-liquid. recent devices can go up to 8 v, which can heat the e-liquid significantly more than earlier generations. fourth-generation fourth-generation pv. a fourth-generation e-cigarette became available in the united states in 2014. fourth-generation e-cigarettes can be made from stainless steel and pyrex glass, and contain very little plastics. included in the fourth-generation are sub ohm tanks and temperature control devices. the e-cigarette user can breathe in large puff volumes, which results in a significant usage of e-liquid per puff. usually used by experienced e-cigarettes users. atomizer and tank inside view of an e-cigarette atomizer with the coil (heating element). an atomizer consists of a small heating element that vaporizes e-liquid and a wicking material that draws liquid onto the coil. along with a battery and e-liquid the atomizer is the main component of every personal vaporizer. when activated, the resistance wire coil heats up and vaporizes the liquid, which is then inhaled by the user. the electrical resistance of the coil, the voltage output of the device, the airflow of the atomizer and the efficiency of the wick all affect the vapor coming from the atomizer. they also affect the vapor quantity or volume yielded. atomizer coils made of kanthal usually have resistances that vary from 0.4ω (ohms) to 2.8ω. coils of lower ohms have increased vapor production but could risk fire and dangerous battery failures if the user is not knowledgeable enough about electrical principles and how they relate to battery safety. wicking materials vary from one atomizer to another. "rebuildable" or "do it yourself" atomizers can use silica, cotton, rayon, porous ceramic, hemp, bamboo yarn, oxidized stainless steel mesh and even wire rope cables as wicking materials. cartomizers a 45mm length, extra-long cartomizer. a "cartomizer" (a portmanteau of cartridge and atomizer.) or "carto" consists of an atomizer surrounded by a liquid-soaked poly-foam that acts as an e-liquid holder. they can have up to 3 coils and each coil will increase vapor production. the cartomizer is usually discarded when the e-liquid starts to taste burnt, which usually happens when the e-cigarette is activated with a dry coil or when the cartomizer gets consistently flooded (gurgling) because of sedimentation of the wick. most cartomizers are refillable even if not advertised as such. cartomizers can be used on their own or in conjunction with a tank that allows more e-liquid capacity. the portmanteau word "cartotank" has been coined for this. when used in a tank, the cartomizer is inserted in a plastic, glass or metal tube and holes or slots have to be punched on the sides of the cartomizer so liquid can reach the coil. clearomizers ego style e-cigarette with a top-coil clearomizer. silica fibers are hanging down freely inside of the tank, drawing e-liquid by capillary action to the coil that is located directly under the mouthpiece. box mod e-cigarette fitted with a rebuildable dripping atomizer (rda). the clearomizer was invented in 2009 that originated from the cartomizer design. it contained the wicking material, an e-liquid chamber, and an atomizer coil within a single clear component. this allows the user to monitor the liquid level in the device. clearomizers or "clearos", are like cartotanks, in that an atomizer is inserted into the tank. there are different wicking systems used inside clearomizers. some rely on gravity to bring the e-liquid to the wick and coil assembly (bottom coil clearomizers for example) and others rely on capillary action or to some degree the user agitating the e-liquid while handling the clearomizer (top coil clearomizers). the coil and wicks are typically inside a prefabricated assembly or "head" that is replaceable by the user. clearomizers are made with adjustable air flow control. tanks can be plastic or borosilicate glass. some flavors of e-liquid have been known to damage plastic clearomizer tanks. rebuildable atomizers a view of the rda deck showing the wicks and coils, e-liquid is dripped into a hopper where the wicks rest as well as atop the coil assembly. a rebuildable atomizer (rba) is an atomizer that allows the user to assemble or "build" the wick and coil themselves instead of replacing them with off-the-shelf atomizer "heads". they are generally considered advanced devices. they also allow the user to build atomizers at any desired electrical resistance. these atomizers are divided into two main categories; rebuildable tank atomizers (rtas) and rebuildable dripping atomizers (rdas). rebuildable tank atomizers rtas have a tank to hold liquid that is absorbed by the wick. they can hold up to 4 ml of e-liquid. the tank can be either plastic, glass, or metal. one form of tank atomizers was the genesis style atomizers. they can use ceramic wicks, stainless steel mesh or rope for wicking material. the steel wick must be oxidized to prevent arcing of the coil. another type is the sub ohm tank. these tanks have rebuildable or rba kits. they can also use coil heads of 0.2 ohm, 0.4 ohm, and 0.5 ohm. these coil heads can have stainless steel coils. rebuildable dripping atomizers rdas are atomizers where the e-liquid is dripped directly onto the coil and wick. the common nicotine strength of e-liquids used in rda's is 3 mg and 6 mg. liquids used in rda's tend to have more vegetable glycerin. a fully saturated wick can give you as many as 10-20 puffs. they typically consist only of an atomizer "building deck", commonly with three posts with holes drilled in them, which can accept one or more coils. the user needs to manually keep the atomizer wet by dripping liquid on the bare wick and coil assembly, hence their name. resistance elements kanthal wire is used in rda's, rba's, rta's, in addition to clearomizers, tanks, and cartomizers. nickel wire or titanium wire can be used for temperature control. squonk mods the origins of a squonk mod bottom-feeding system go as far back as 2009. a member of the e-cigarette forum (ecf) named "carlos49" was largely credited with developing the first squonker available in the marketplace. squonk mods differ from other mod boxes with their construction. squonk mods have a 510 connection that have been modified with the use of an e-liquid bottle placed inside the mod. the user squeezes an e-liquid bottle through an opening in the device to send e-liquid through a tube into the attached atomizer. extra liquid goes back into the bottle when it is unsqueezed. pod mods juul e-cigarette with pods. bravo nicotine salt e-liquids. pod mods heat up a liquid containing nicotine, flavors, and other ingredients that creates an aerosol. pod mods are lightweight, portable, small, and reusable. pod mods do not require pushing a button. a pod mod does not require much of a learning curve. with the majority of pod mods, users can just open their new package, put a pod into the device, and begin vaping. they are charged using a usb port. there are numerous pod mods in the marketplace and there are many kinds of pod mods. the three categories for the different kinds of pod mods are an open system, a closed system, or those that use both. pod mods come in varying colors and flavors. many devices rely on replaceable liquid pods that may contain propylene glycol, glycerin, benzoic acid, nicotine, and artificial flavors. some pod mods can be refillable, with flavors such as cotton candy, donut cream, and gummy bear. pod mods that contain tetrahydrocannabinol (thc), the primary psychoactive chemical of cannabis, are being sold. pod mods can look like usb flash drives, cell phones, credit card holders, and highlighters. because pod mods are small and generate less aerosol, it makes it easy to hide them. there are pod mods that can be concealed in the palm of a person's hand. later-generation pod mods are small like a sharpie pen. pod mods cost about half as much as larger e-cigarettes. the latest generation of e-cigarettes, "pod products", such as juul, have the highest nicotine content (59 mg/ml), in protonated salt, rather than the free-base nicotine form found in earlier generations, which makes it easier for less experienced users to inhale. pod mods deliver higher levels of nicotine than regular e-cigarettes. one nicotine pod, in terms of nicotine, is roughly equivalent to one pack of regular cigarettes. the labels on products state pods contain 59 mg/ml of nicotine, but the levels can be considerably greater such as 75 mg/ml of nicotine. some pod mods contained greater levels of nicotine than juul which were as high as 6.5%. in june 2015, juul introduced a pod mod device. british american tobacco told to the verge in 2018 that "they've been incorporated in our vuse e-liquid in the us since 2012." research on nicotine salts is limited. tests show that the pod mods juul, bo, phix, and sourin contain nicotine salts in a solution with propylene glycol and glycerin. a nicotine base and a weak acid such as benzoic acid or levulinic acid is used to form a nicotine salt. benzoic acid is the most used acid to create a nicotine salt. a free-base nicotine solution with an acid reduces the ph, which makes it possible to provide higher levels of nicotine without irritating the throat. nicotine salts are thought to amplify the level and rate of nicotine