referring to fig1 and 2 ( which are not to scale ), liquid which it is desired to atomise is supplied via an insulating pipe 2 to an outlet 6 for the liquid . in the apparatus illustrated the outlet 6 is a metal capillary tube and is thus electrically conducting . the end of the outlet provides a spraying edge from which the liquid is sprayed . spaced from and in front of the spraying edge 7 is a shield electrode 8 . the outlet 6 and the shield electrode 8 are connected to respective terminals of a high voltage generator 10 which in use charges the spraying edge 7 to a high voltage of one polarity , preferably positive , with respect to the shield electrode 8 . the voltage between the spraying edge 7 and the shield electrode 8 produces a sufficient electrode field strength between them to draw a cone 11 of liquid from the spraying edge . the liquid leaving the spraying edge becomes charged , negative charge being conducted away by the conducting spraying edge 7 , leaving a net positive charge on the liquid . the charge on the liquid produces internal repulsive electrostatic forces which overcome the surface tension of the liquid forming a cone of liquid from the tip of which issues a ligament 12 . at a distance from the spraying edge , the mechanical forces produced on the ligament due to travelling through the air cause it to break up into charged droplets of closely similar size . the shield electrode 8 replaces the field intensifying electrode described in the above mentioned british patent specification no . 1 , 569 , 707 . in order to produce the very small droplets necessary for an inhaler , the electric field strength produced between the spraying edge and the shield electrode needs to be high . for example , the voltage between the shield electrode 8 and the spraying edge may be in the region of 5 kv , whilst the spacing between them may be less than 5 mm , say , 2 mm . it is known that air ionises differently at the different polarities and that the threshold is higher for positive potentials . in order to produce the small droplets referred to , the electric field strength is found to be so high that if the spraying edge 7 is charged negative relative to the shield electrode 8 , there is a substantial risk of corona from the spraying edge and / or the cone 11 which interferes with or destroys the stability of the ligament . we have found that suitably small droplets can be produced when the spraying edge is charged positive , without the risk of corona from the spraying edge 7 or cone 11 . the formulations used in inhalers have a resistivity which is much lower than that usually used for electrostatic spraying . in most prior art applications , say , paint spraying or spraying agricultural pesticides , in order to obtain a useful flow rate , the liquids to be sprayed usually have resistivities in the range 10 6 to 10 10 ohm cm . the formulations which might be used in an inhaler are expected to have resistivities in the range 5 × 10 3 to 10 8 ohm cm . although spraying of liquids which have a resistivity at the low end of the range is known from , say , u . s . patent specification no . 1 , 958 , 406 it only works at low flow rates which have not been of much practical use . if the flow rate is too high , the ligament becomes unstable . we have found , however , that a stable ligament can be produced , and the apparatus can be made to spray satisfactorily , if the flow rate is low enough , a situation which is appropriate to inhalers , but not perhaps to agricultural or paint sprayers . it is likely that suitable formulations will have resistivities in the range 10 5 to 10 7 ohm cm . given that a stable spray can be produced , the lower resistivities assist in the production of a small droplet size . all other parameters equal , we find that the lower the resistivity , the smaller the droplet size . the shield electrode has an orifice 14 aligned with the spraying edge 7 and sufficiently large that the ligament or the droplets pass through , dependent on whether the ligament breaks up before or after the shield electrode , to produce a spray of droplets in a chamber 15 beyond the shield . if the orifice were too small the droplets or ligament would deposit on the shield electrode 8 . previous expectations as expressed in the above mentioned british patent specification no . 1 , 569 , 707 would have been that with the shield electrode positioned downstream of the atomising tip of the ligament , droplets would deposit on the shield electrode even with an orifice very substantially larger than illustrated . a small orifice is required in the present apparatus for reasons explained below . in order to produce a spray which is inhalable , the droplets which issue through the orifice 14 must be discharged . this is effected by a discharge electrode in the form of a needle 16 . in the embodiment illustrated , the needle is directly in the path of the spray . in alternative embodiments , one or more discharge electrodes may be positioned out of the direct path of the spray . the discharge electrode is connected to the high voltage generator 10 which , in use , charges the discharge electrode 16 to a high potential relative to the shield electrode 8 and of opposite polarity to that of the spraying edge , in this case negative . the shield electrode may be provided with a connection to earth , perhaps via a leakage path through the user of the inhaler , or may be left floating . the discharge electrode 16 is driven to a sufficiently high voltage relative to the shield electrode 8 as to produce a corona discharge . the negative ions so produced discharge droplets in the spray issuing through the orifice 14 . the distinction between charged droplets and discharged droplets is very obvious visually . any droplets remaining charged in the spray , are highly mobile in a predictable path . discharged particles appear as a cloud or smoke which drifts unpredictably in the air currents . if a prior art field intensifying electrode were used in the place of the present shield electrode , there would be considerable difficulty in discharging all the droplets in the spray . why this is so can be understood by considering what happens as the voltage on the discharge electrode is increased from a voltage insufficient to cause ionic discharge . fig4 shows an enlarged and schematic view of what would happen . a prior art field intensifying or field adjusting electrode is illustrated at 27 . at the larger scale used in fig4 the ligament 12 can be seen oscillating at 28 due to mechanical disturbance of passing through the air . the ligament breaks up into droplets which separate into a spray bounded approximately by a cone indicated in broken outline at 30 . within the spray , the charged droplets are highly mobile in predictable paths generally to the right of fig4 . at a threshold the voltage is sufficiently high that the electric field strength around the sharp tip of the needle ionises the surrounding air leaving free negative ions . these discharge surrounding droplets in an area bounded , say , by broken line 32 . the discharged droplets are easily identifiable visually . they lose their predictable mobility , becoming a drifting smoke which is very distinct from the charged droplets . as the voltage is increased droplets are discharged further from the discharge electrode , so that more of the spray is discharged as indicated by , say , broken line 34 . when the voltage applied to the needle is sufficiently high that the boundary of the discharged droplets , indicated by broken line 36 , reaches the edge of the spray cone 30 , the spray ( which is travelling to the right in fig3 ) would be completely discharged . unfortunately , at this point the corona uncontrollably jumps to the cone 11 and / or the spraying edge 7 , which discharges the cone 11 . since it was the charge on the liquid which overcame the surface tension thereof to form the cone 11 and repel the ligament 12 therefrom , discharging the cone 11 destroys the spray . the shield electrode 8 is arranged to shield the spraying edge 7 and the cone 11 from the corona thus enabling all the droplets in the spray to be discharged without danger of the cone being discharged . to achieve this , the orifice 14 must not be too large otherwise corona will find its way through . as mentioned above , the orifice must not be too small either , otherwise the droplets will not spray through the orifice but will deposit on the shield electrode . we have found it entirely possible to balance these conflicting requirements so that the orifice can be at the same time neither too large nor too small . with a flow rate of about 40 microliters per minute , we found that the apparatus could be made to work with a hole 14 having a diameter in the range 2 mm to 1 cm . at the small end of the range , there was a greater tendency to spray onto the shield electrode . at the large end of the range , there was a greater tendency for the corona to leak through the hole to the cone 11 . complete discharge of the spray can be ensured by adjusting the position of the needle 16 and the voltage applied thereto . we found no need in the arrangement illustrated for the voltage to be more than 10 kv and that it could , indeed , be well below that . the overall dimensions of the shield electrode must be sufficient to prevent corona reaching the cone 11 or spraying edge 6 round the outside of the electrode . the shield electrode 8 may be metallic but need not necessarily be of such a good conductor as that . what is required is that the shield electrode should be sufficiently conducting to remove any charge which may accumulate due to the ionic discharge . the outlet 6 , shield electrode 8 and discharge electrode 16 are contained within a capsule 50 having a volume of about 200 ml , defining the walls of the chamber 15 . in order to reduce interference with the electrical fields produced between the spraying edge and the shield electrode on the one hand , and the discharge electrode and the shield electrode on the other hand , the capsule is made of an insulating or semi - insulating material . polyethylene plastics is an example of a very highly insulating material . in order that any charge received by the capsule may leak away , it may be preferred to use a less insulating material for example polycarbonate , polyethylene terephthalate or polyacetal . the shield electrode may be cup shaped extending inside the walls towards the discharge electrode , or may form part of the walls . two holes 52 and 54 provide an air passage through the chamber 15 transverse to the direction in which the spraying edge sprays . the discharged spray can thus be removed in an airstream across the chamber and inhaled . the airstream may be produced by the user inhaling or , if necessary or preferred , by a fan ( not shown ). the discharge electrode 16 has a sharp tip which just projects into the chamber 15 . the wall containing the discharge electrode is concave with respect to the chamber . both these features are intended to reduce the volume of the chamber which is behind ( to the right in fig2 ) of the corona discharge . it is desired that the corona discharge sweeps as much of the chamber 15 as possible without being so directed as to charge the walls of the chamber which would be both wasteful and could charge the walls so interfering with the formation of corona as mentioned above . to facilitate the user inhaling the discharged spray , the air passage communicates with a face mask 36 as shown in fig1 . a non return valve , for example a flap ( not shown ), may be placed between the face mask and the capsule to prevent exhaled breath blowing the discharged droplets out through the hole 54 . also shown in fig1 is a supplies pack 38 , which in this case is shown separate . the liquid supply tube 2 and the high voltage electrical leads are bundled together as indicated schematically . a mobile emphysema patient , for example , can put the supplies pack in a pocket and move about whilst using the mask . the supplies pack contains the high voltage generator 10 and delivery means 40 for supplying metered amounts of the liquid to be inhaled . the flow rate of the liquid supplied to the outlet 6 is required to be very accurate . accuracy is required both so that the patient gets the correct dose , and so that the droplet size remains accurate . in order to achieve the required accurate flow rate , the delivery means 40 comprises a syringe 42 containing the liquid . the syringe is replaceable when empty . the syringe has a plunger 44 , which is driven directly by a friction wheel 45 . this is driven by a stepping motor and reduction gearbox 46 controlled by an electrical controller 48 which may include means for setting the rate , or alternatively the rate may be fixed . this general arrangement is already used in a metered pump for administering successive doses of insulin . the pump is manufactured by muirhead vactric components ltd . of beckenham , kent . although the outlet capillary tube 6 is metallic in the above example , it is possible to use an insulating tube , especially when the liquid to be atomised has a resistivity towards the lower end of the range . in this case an electrode contacts the liquid upstream of the spraying edge . the liquid is itself sufficiently conducting to carry the charge to the spraying edge so to define the electric field between the spraying edge and the shield electrode 8 . the lower the resistivity of the liquid the further upstream contact can be made with the liquid . although illustrated as one integral piece the capsule 50 may be formed as two separable parts to facilitate cleaning . a further form of spraying edge is illustrated in fig3 . this spraying edge comprises an outer insulating tube 18 tapered externally at one end 20 . a conducting core 22 is connected to the high voltage generator 10 by a lead 23 . the core 22 tapered to a point 24 at one end . the point 24 projects slightly beyond the outer insulating tube so defining an annular orifice 26 therewith . the arrangement produces a single ligament from the tip of the projecting point of the core . all the examples of spraying edges described above are arranged to produce a single ligament principally because of the very low flow rate which may be required in the particular application of an inhaler . in applications where higher flow rates may be required , it may be appropriate to use a spraying edge which produces a plurality of ligaments . one form of spraying edge which produces a plurality of ligaments is a linear nozzle ( not shown ). in this form liquid is fed to a a linear edge at which the intense electric field is formed . the linear edge may be fed with liquid from a slot at or spaced from the edge . a linear nozzle is illustrated in british patent specification no . 1569707 . if the edge is plane , ligaments from along its length at intervals determined by various factors including the field strength and the flow rate . it is possible to position the ligaments to some extent by means of irregularities in the edge , for example teeth , which provide local intensification of the field from which the ligaments issue . to illustrate the use of a linear nozzle , fig2 could still be considered a cross section through the apparatus , the outlet 6 , shield electrode 8 and discharge electrode 16 all extending linearly at right angles to the plane of the paper . in effect the outlet would be a slot feeding a linear edge . it is possible to space the slot back from the edge , as would be the case if the arrangement of fig3 were considered to be a section through a linear nozzle . the spraying edge would then be a linear edge instead of a point described above , and the annular orifice would be a slot above and below the edge . the edge could be fed from a slot on just one side . in such arrangements the orifice 14 in the shield electrode is in the form of a slot and the discharge electrode is either in the form of an edge or a row of discrete points as that illustrated . formulations suitable for use with an inhaler embodying the invention , are likely to have a resistivity in the range 5 × 10 3 to 10 8 ohm cm . predominantly aqueous formulations are not completely satisfactory , since the droplet size is so small that evaporation takes place very quickly . water also has a high surface tension which makes it difficult to spray . suitably the formulation comprises an acceptable organic diluent and the amount of water , if present , comprises not more than about 50 % of the total diluent , more suitably less than 20 % and preferably less than 10 %. the formulation consists of a suitable , pharmaceutically acceptable , solvent e . g . dimethyl isosorbide , glycerol , propylene glycol and polyethylene glycol of average molecular weight up to about 600 admixed with water or ethanol . in addition the formulations may contain a suitable pharmaceutically acceptable surfactant e . g . polyethoxy - ethylated castor oils (&# 34 ; cremophors &# 34 ;), polyoxyethylene - polyoxypropylene block copolymers (&# 34 ; pluronics &# 34 ;, &# 34 ; synperonics &# 34 ;), polyoxyethylene sorbitan derivatives (&# 34 ; tweens &# 34 ;), polyoxyethylene oleyl ethers (&# 34 ; brijs &# 34 ;) and sorbitan esters of fatty acids (&# 34 ; spans &# 34 ;). such materials are preferably present at not more than 1 % concentration . an inhaler embodying the invention is suitable for the administration to a patient of any drug which can be administered via the lungs , either to have a direct effect on the lungs , for example for the treatment of asthma , emphysema or bronchitis , or for absorption from the lung into the bloodstream in order to produce a systemic therapeutic effect elsewhere in the body . examples of drugs which have a direct effect on the lung for the treatment of asthma , emphysema or bronchitis are bronchospasm relaxants such as adrenaline , isoprenaline , orciprenaline , ephedrine , phenylephrine , diphenhydramine , terbutalene , isoetharine , etafedrine , methoxyphenamine , theophylline , aminophylline , salbutamol , sodium cromoglycate , ipratropium bromide , beclomethasone , betamethasone valerate , fenoterol reproterol , pirbuterol , budesonide , ketotifen and compounds as defined and claimed in published european patent specification no . 189 305 , particularly those compounds defined in claims 4 , 5 and 6 . examples of drugs which can be administered systemically via the lungs include polypeptide drugs , for example luteinising hormone - releasing hormone , and synthetic analogues thereof . suitably the active ingredient is in the formulation in a concentration range of 0 . 1 to 20 %, and preferably 5 to 10 %, but the required concentration depends , naturally , upon the potency of the particular drug being used .
apparatus , for example an inhaler , is disclosed in which a spray is formed electrostatically from an outlet 6 to achieve uniform droplet size . the spray is then discharged for its intended use -- in the example inhaling . the spray is discharged completely by corona produced by a sharp discharge electrode 16 which is charged to a polarity opposite that of the liquid . in order to prevent the corona from spoiling the formation of the spray itself , the liquid of the outlet 6 is protected by a neutral shield electrode 8 . the spray issues through a hole 14 in the shield electrode . the hole is sufficiently small to prevent corona getting through , yet sufficiently large to allow the spray through .